Cuando intento instalar un archivo apk firmado, el instalador de la aplicación dice "Aplicación no instalada".
Le sucede a todas las aplicaciones que hice. Incluso si creo un nuevo almacén de claves o si configuro el modo de compilación para depurar.
Aunque funciona si lo instalo a través de Android Studio. Pero si necesito compartirlo con mis amigos, no puedo.
Traté de buscar una pregunta que pueda apelar mi situación en este sitio, pero no he encontrado ninguna.
Mi aplicación está funcionando y (supuestamente) firmada. No puedo encontrar ninguna razón para que no funcione.

Por ejemplo, este es el código para la aplicación JustJava que hice con el curso de desarrollo de Android en Udacity.

El activity_main.xml es:

<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">


<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp">

    <EditText
        android:id="@+id/name"
        android:hint="@string/name_field"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="text"
        android:layout_marginBottom="16dp"/>

    <TextView
        style="@style/HeaderTextStyle"
        android:text="@string/toppings" />


    <CheckBox
        android:id="@+id/topping_whipped_cream"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="16dp"
        android:paddingLeft="24dp"
        android:text="@string/topping_whipped_cream" />
    <CheckBox
        android:id="@+id/topping_chocolate"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="16dp"
        android:layout_marginLeft="16dp"
        android:paddingLeft="24dp"
        android:text="@string/topping_chocolate" />

    <TextView
        style="@style/HeaderTextStyle"
        android:text="@string/quantity" />

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:padding="16dp">

        <Button

            android:id="@+id/decrement"
            android:layout_width="48dp"
            android:layout_height="wrap_content"
            android:onClick="decrement"
            android:text="@string/minus" />

        <TextView
            android:id="@+id/quantity_text_view"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:paddingBottom="16dp"
            android:paddingLeft="16dp"
            android:paddingRight="16dp"
            android:paddingTop="16dp"
            android:text="@string/quantity_num"
            android:textColor="@android:color/black"
            android:textSize="18sp" />

        <Button
            android:id="@+id/increment"
            android:layout_width="48dp"
            android:layout_height="wrap_content"
            android:onClick="increment"
            android:text="@string/plus" />

    </LinearLayout>


    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal">


        <Button
            android:id="@+id/order"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="submitOrder"
            android:text="@string/order_btn" />
    </LinearLayout>

</LinearLayout>

Y el Java es:

package com.example.android.justjava;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.CheckBox;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.text.NumberFormat;

/**
 * This app displays an order form to order coffee.
 */
public class MainActivity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}
/**
 * Assigns the name text field
 */


/**
 * This method is called when the order button is clicked.
 */
int quantity = 1;

public void submitOrder(View view) {
    EditText name = (EditText) findViewById(R.id.name);
    String clientName = name.getText().toString();

    Intent submit = new Intent(Intent.ACTION_SEND);
    submit.setData(Uri.parse("mailto:"));
    submit.setType("*/*");
    submit.putExtra(Intent.EXTRA_CC, "[email protected]");
    submit.putExtra(Intent.EXTRA_SUBJECT, getString(R.string.mail_subject_order) + clientName);
    submit.putExtra(Intent.EXTRA_TEXT, createOrderSummary());
    if (submit.resolveActivity(getPackageManager()) != null) {
        startActivity(submit);
    }
}

public void increment(View view) {
    if (quantity < 99) {
        quantity += 1;
        displayQuantity(quantity);
    } else {
        Toast toast = Toast.makeText(getApplicationContext(),
                getString(R.string.over_100_coffees_toast), Toast.LENGTH_LONG);
        toast.show();
    }
}

public void decrement(View view) {
    if (quantity == 1) {
        Toast toast = Toast.makeText(getApplicationContext(), R.string.no_coffees_toast, Toast.LENGTH_SHORT);
        toast.show();
    } else {
        quantity -= 1;
        displayQuantity(quantity);
    }

}

/**
 * This method displays the given quantity value on the screen.
 *
 * @param quant
 */
private void displayQuantity(int quant) {
    TextView quantityTextView = (TextView) findViewById(
            R.id.quantity_text_view);
    quantityTextView.setText("" + quant);
}


/**
 * Creates a visual summary of the order.
 * <p/>
 * quantity is the number of cups of coffee ordered
 */

private String createOrderSummary() {
    /**
     * Assigns the whipped cream checkbox
     */
    CheckBox toppingWhippedCream = (CheckBox) findViewById(R.id.topping_whipped_cream);
    boolean isCheckedWhippedCream = toppingWhippedCream.isChecked();
    /**
     * Assigns the chocolate checkbox
     */
    CheckBox toppingChocolate = (CheckBox) findViewById(R.id.topping_chocolate);
    boolean isCheckedChocolate = toppingChocolate.isChecked();
    //retrieve client's name
    EditText name = (EditText) findViewById(R.id.name);
    String clientName = name.getText().toString();
    /**
     * Inputs the total price
     */
    int pricePerCup = 5;
    if (isCheckedWhippedCream) {
        pricePerCup += 1;
    }

    if (isCheckedChocolate) {
        pricePerCup += 2;
    }
    int totalPrice = pricePerCup * quantity;
    return          //returns the name of the client
            getString(R.string.name_at_order_summary) + clientName + "\n"
                    //return quantity
                    + getString(R.string.quantity_at_order_summary) + quantity + "\n"
                    //return if whipped cream is checked
                    + getString(R.string.topping_whipped_cream_at_order_summary) + isCheckedWhippedCream + "\n"
                    //return if chocolate is checked
                    + getString(R.string.topping_chocolate_at_order_summary) + isCheckedChocolate + "\n"
                    //return total price
                    + getString(R.string.total_price_at_order_summary) + totalPrice + "\n" +
                    getString(R.string.thank_you_at_order_summary);
}


}

Mainfest:

<?xml version="1.0" encoding="utf-8"?>

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

¡Gracias!