Android: How to listen to hardware key input on background?

Question

I am working on an app that is going to trigger an alarm when volume key is pressed and hold for some time. I have research and it seems like some people claim it is impossible to get input feedback from a service, while others claim it is possible with an extended class of ContentObserver or using the music media api, however I still haven't been able to get it to work.

The following app does exactly what I need, when you activate it, it will take a screen shot when you press and hold the volume and power buttons after a certain period of time regardless of whether is closed, locked, etc..

https://play.google.com/store/apps/details?id=com.enlightment.screenshot&hl=en

My question is how can I do the above with a service class.

Code:

This is my ContentObserver class:

int previousVolume;
Context context;

public ContentListener(Context c, Handler handler) {
    super(handler);
    context=c;

    AudioManager audio = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
    previousVolume = audio.getStreamVolume(AudioManager.STREAM_MUSIC);
}

@Override
public boolean deliverSelfNotifications() {
    return super.deliverSelfNotifications();
}

@Override
public void onChange(boolean selfChange) {
    super.onChange(selfChange);

    AudioManager audio = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
    int currentVolume = audio.getStreamVolume(AudioManager.STREAM_MUSIC);

    int delta=previousVolume-currentVolume;

    if(delta>0)
    {
        Logger.log(Logger.LOG, "Volume Down");
        previousVolume=currentVolume;
    }
    else if(delta<0)
    {
        Logger.log(Logger.LOG, "Volume Up");
        previousVolume=currentVolume;
    }
}

This is my Service class:

private ContentListener listener;

@Override
public void onCreate() {

    Logger.log(Logger.LOG, "OnCreate");

    listener = new ContentListener(this, new Handler());
    getApplicationContext().getContentResolver().registerContentObserver(
            android.provider.Settings.System.CONTENT_URI, true, listener);
}

@Override
public void onDestroy() {

    Logger.log(Logger.LOG, "OnDestroy");
    getApplicationContext().getContentResolver().unregisterContentObserver(listener);
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) { return Service.START_STICKY; }
@Override
public IBinder onBind(Intent intent) { return null; }