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/**
*   First, extend NotificationListener and override necessary methods
*/
public class NotificationListener extends NotificationListenerService{
    // override methods here
}
/**
*   Next, add all required permissions and declarations to your manifest
*/
<uses-permission 
    android:name="android.permission.WRITE_SECURE_SETTINGS"
    tools:ignore="ProtectedPermissions"
/>
<service
    android:name=".NotificationListener"
    android:permission="android.permission.BIND_NOTIFICATION_LISTENER_SERVICE"
    android:enabled="true">
        <intent-filter>
            <action android:name="android.service.notification.NotificationListenerService"/>
        </intent-filter>
</service>
/**
*   In your activity, check if service is enabled in system, if not enable it
*   NOTE: Service can be enabled programmatically but then must be installed
*   as a system app
*/
// Button 1
readServicesButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        ContentResolver contentResolver = context.getContentResolver();
        String enabledListeners = Settings.Secure.getString(
            contentResolver, "enabled_notification_listeners");
        Log.d("SERVICES", enabledNotificationListeners);
    }
});
// Button 2
writeServiceButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        ContentResolver contentResolver = context.getContentResolver();
        Settings.Secure.putString(
        contentResolver, "enabled_notification_listeners", 
        enabledListeners + ":" 
        + "your.package.name/your.package.name.NotificationListener");
        Log.d("SERVICES", "Added!");
    }
});
// NOTE: Don't forget the ":" between previous enabled services and appending yours