Android close running apps

I have developed a service to automatically detect whether an app has started. If the user forbid this app to start (could be chosen from a list of apps), a dialog will be shown.

public class AppStartReceiver extends Service {
    private static final int DELAY_IN_MS = 2000;
    private List<String> forbiddenApps = new ArrayList<String>();
    private BroadcastReceiver screenReceiver = null;
    private Timer timer;
    private static boolean screenOn = true;

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {

        //register screen on/off receiver
        registerScreenReceiver();

        if (forbiddenApps.isEmpty()) {
            DatabaseHelper db = new DatabaseHelper(this);
            forbiddenApps = db.getAllApps();
        }

        //start listening for app starts
        listenForAppStarts();

        return START_STICKY;
    }

    @Override
    public void onDestroy() {
        unregisterReceiver(screenReceiver);
        timer.cancel();
        super.onDestroy();
    }

    /**
     * start listening for apps to start
     * only foreground apps are being watched
     */
    private void listenForAppStarts() {
        final ActivityManager actMgr = (ActivityManager)getSystemService(Context.ACTIVITY_SERVICE);
        timer = new Timer();
        timer.scheduleAtFixedRate(new TimerTask() {

            public void run() {
                final String foregroundApp = actMgr.getRunningTasks(1).get(0).topActivity.getPackageName();
                if (forbiddenApps.contains(foregroundApp)) {
                    //forbidden app detected => show dialog
                    Intent dialogIntent = new Intent(getBaseContext(), AlertActivity.class);
                    dialogIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                    startActivity(dialogIntent);
                }

                // stop if user turns screen off
                if(!screenOn) {
                    timer.cancel();
                }
            }
        }, 0, DELAY_IN_MS);
    }

    @Override
    public IBinder onBind(Intent arg0) {
        return null;
    }

    /**
     * register screen receiver
     * gets callback from it
     */
    private void registerScreenReceiver() {
        // create new receiver
        screenReceiver = new BroadcastReceiver(){
            @Override
            public void onReceive(Context context, Intent intent) {
                if (intent.getAction().equals(Intent.ACTION_SCREEN_OFF)) {
                    Log.i("ScreenReceiver", "Screen off");
                    screenOn = false;
                } else if (intent.getAction().equals(Intent.ACTION_SCREEN_ON)) {
                    Log.i("ScreenReceiver", "Screen on");
                    listenForAppStarts();
                    screenOn = true;            
                }
            }
        };

        //register receiver
        IntentFilter filter = new IntentFilter(Intent.ACTION_SCREEN_ON);
        filter.addAction(Intent.ACTION_SCREEN_OFF);
        registerReceiver(screenReceiver, filter);
    }

}

I works perfectly, but I don't know how good it is (does it drain too much battery?). Also, I'd like to know if my programming style is ok.