Making a foreground service run until it's stopped

My goal is to have an app run in the background even if the screen is switched off. Like a music player. Now everyone claims that you needed a foreground service for that. I am using a class extending Worker for this:

public class WorkerKlasse extends Worker {
    String CHANNEL_ID = "1";
    Context context;
    public WorkerKlasse(
            @NonNull Context context,
            @NonNull WorkerParameters parameters) {
        super(context, parameters);
        this.context = context;
        notificationManager = (NotificationManager)
                context.getSystemService(NOTIFICATION_SERVICE);
    }
    private NotificationManager notificationManager;
    @RequiresApi(Build.VERSION_CODES.O)
    private void createChannel() {
            int importance = NotificationManager.IMPORTANCE_DEFAULT;
            NotificationChannel channel = new NotificationChannel(CHANNEL_ID, "Kanal", importance);
            channel.setDescription("Beschreibung");
            channel.setSound(null,null);
            // Register the channel with the system; you can't change the importance
            // or other notification behaviors after this

                NotificationManager notificationManager = (NotificationManager)context.getSystemService(NOTIFICATION_SERVICE);
            notificationManager.createNotificationChannel(channel);
    }
    @NonNull
    private ForegroundInfo createForegroundInfo(@NonNull String progress) {
        // Build a notification using bytesRead and contentLength

        Context context = getApplicationContext();
        // This PendingIntent can be used to cancel the worker
        PendingIntent intent = WorkManager.getInstance(context)
                .createCancelPendingIntent(getId());

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            createChannel();
        }

        Notification notification = new NotificationCompat.Builder(context, CHANNEL_ID)
                .setContentTitle("Test")
                .setTicker("Test2")
                .setSound(null)
                .setSmallIcon(R.drawable.ic_launcher_background)
                .setOngoing(true)
                // Add the cancel action to the notification which can
                // be used to cancel the worker
                .addAction(android.R.drawable.ic_delete, "A", intent)
                .build();

        return new ForegroundInfo(1,notification);
    }

    @NonNull
    @Override
    public Result doWork() {
        int testint = getInputData().getInt("testint",0);
        ...
        String progress = "Beginn";
        setForegroundAsync(createForegroundInfo(progress));
        //do something, in my case
        mediaPlayer = MediaPlayer.create(getApplicationContext(), R.raw.testsound);
        mediaPlayer.start();
        try {
            Thread.sleep(testint);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }            
        return Result.success();
    }

}

(before return Result.success(); the foreground service should stop) and I start with this in MainActivity:

WorkRequest myUploadWork =
        new OneTimeWorkRequest.Builder(WorkerKlasse.class)
                .setInputData(
                        new Data.Builder()
                                .putInt("testint",testint)
                                //...
                                .build()
                )
                .build();

WorkManager
        .getInstance(MainActivity.this)
        .enqueue(myUploadWork);

Yet it does not keep running when the screen goes dark or I tap once on the power button to make it go dark. Only after entering the passcode to get into the phone, it continues. The icon on the top to show it's a foreground service is shown, though.