Send a notification when the app is closed

You can use this service all you need to do is Start this service onStop() in your activity lifecycle. With this code:
startService(new Intent(this, NotificationService.class));
then you can create a new Java Class and paste this code in it:

public class NotificationService extends Service {

    Timer timer;
    TimerTask timerTask;
    String TAG = "Timers";
    int Your_X_SECS = 5;


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

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

        startTimer();

        return START_STICKY;
    }


    @Override
    public void onCreate() {
        Log.e(TAG, "onCreate");


    }

    @Override
    public void onDestroy() {
        Log.e(TAG, "onDestroy");
        stoptimertask();
        super.onDestroy();


    }

    //we are going to use a handler to be able to run in our TimerTask
    final Handler handler = new Handler();


    public void startTimer() {
        //set a new Timer
        timer = new Timer();

        //initialize the TimerTask's job
        initializeTimerTask();

        //schedule the timer, after the first 5000ms the TimerTask will run every 10000ms
        timer.schedule(timerTask, 5000, Your_X_SECS * 1000); //
        //timer.schedule(timerTask, 5000,1000); //
    }

    public void stoptimertask() {
        //stop the timer, if it's not already null
        if (timer != null) {
            timer.cancel();
            timer = null;
        }
    }

    public void initializeTimerTask() {

        timerTask = new TimerTask() {
            public void run() {

                //use a handler to run a toast that shows the current timestamp
                handler.post(new Runnable() {
                    public void run() {

                        //TODO CALL NOTIFICATION FUNC
                        YOURNOTIFICATIONFUNCTION();

                    }
                });
            }
        };
    }
}

After this you only need to combine the service with the manifest.xml:

<service
            android:name=".NotificationService"
            android:label="@string/app_name">
            <intent-filter>
                <action android:name="your.app.domain.NotificationService" />

                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </service>