Trigger a function inside Background service when any app is launched on mobile?

Question

I am developing App-Locker app and i have background service which is running. But i want the exact solution to trigger one function inside that background service when any application is launch or open in mobile. I am using the below service which runs continuously.

public class NotificationService  extends Service {
public NotificationService() {
}

@Override
public IBinder onBind(Intent intent)
{
    Log.e("Noftication serveric ", " Onbinddddd");
    return null;
}

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

    return START_STICKY;
}

@Override
public void onDestroy()
{
    super.onDestroy();
      //Starting Broadcast on destroy method, where this service is called again to run
    Intent restartService = new Intent("RestartService");
    sendBroadcast(restartService);

}

public void Trigger_Me_If_Any_App_Launch()   
 {
      //This function should be called if any app launch
 }

}