Android O - Single line Notification - like the "Android System - USB charging this device"

To display a compact single line notification like the charging notification, you have to create a Notification Channel with priority to IMPORTANCE_MIN.

@TargetApi(Build.VERSION_CODES.O)
private static void createFgServiceChannel(Context context) {
   NotificationChannel channel = new NotificationChannel("channel_id", "Channel Name", NotificationManager.IMPORTANCE_MIN);
   NotificationManager mNotificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
   mNotificationManager.createNotificationChannel(channel);
}

And then create an ongoing notification like that:

public static Notification getServiceNotification(Context context) {
   NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(context, "channel_id");
   mBuilder.setContentTitle("One line text");
   mBuilder.setSmallIcon(R.drawable.ic_notification);
   mBuilder.setProgress(0, 0, true);
   mBuilder.setOngoing(true);
   return mBuilder.build();
}

NOTE

Please note that I've tested it with an IntentService instead of a Service, and it works. Also I've just checked setting a Thread.sleep() of 15 seconds and the notification is showing perfectly until the IntentService stops itself.

There are some images (sorry some texts are in Spanish, but I think the images are still useful):

And if you drag down and opens the notification, it's shown as follows:

EXTRA

If you notice that Android System shows a notification indicating all apps which are using battery (apps with ongoing services), you can downgrade the priority of this kind of notifications and it will appear as one line notifications like the charging notification.

Take a look at this:

Just long click on this notification, and select ALL CATEGORIES:

And set the importance to LOW:

Next time, this "battery consumption" notification will be shown as the charging notification.

You need to set the Notification priority to Min, the Notification Channel importance to Min, and disable showing the Notification Channel Badge.

Here's a sample of how I do it. I've included creating the full notification as well for reference

private static final int MYAPP_NOTIFICATION_ID= -793531;

NotificationManager notificationManager = (NotificationManager) context
        .getSystemService(Context.NOTIFICATION_SERVICE);

String CHANNEL_ID = "myapp_ongoing";
CharSequence name = context.getString(R.string.channel_name_ongoing);

if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
    NotificationChannel channel = new NotificationChannel(CHANNEL_ID, name, NotificationManager.IMPORTANCE_MIN);
    channel.setShowBadge(false);

    notificationManager.createNotificationChannel(channel);
}

NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(
        context, CHANNEL_ID)
        .setSmallIcon(R.drawable.ic_stat_notification_add_reminder)
        .setContentTitle(context.getString(R.string.app_name))
        .setContentText(context.getString(R.string.create_new))
        .setOngoing(true).setWhen(0)
        .setChannelId(CHANNEL_ID)
        .setPriority(NotificationCompat.PRIORITY_MIN);

// Creates an intent for clicking on notification
Intent resultIntent = new Intent(context, MyActivity.class);
...

// The stack builder object will contain an artificial back stack
// for the
// started Activity.
// This ensures that navigating backward from the Activity leads out
// of
// your application to the Home screen.
TaskStackBuilder stackBuilder = TaskStackBuilder.create(context);
// Adds the back stack for the Intent (but not the Intent itself)
stackBuilder.addParentStack(MyActivity.class);
// Adds the Intent that starts the Activity to the top of the stack
stackBuilder.addNextIntent(resultIntent);
PendingIntent resultPendingIntent = stackBuilder.getPendingIntent(0,
        PendingIntent.FLAG_UPDATE_CURRENT);
mBuilder.setContentIntent(resultPendingIntent);

notificationManager.notify(MYAPP_NOTIFICATION_ID, mBuilder.build());

To answer the original question:

There seems to be no built-in way on Android O to get a single line, ongoing notification for a ForegroundService. One could try adding a custom design, but as different phones have different designs for notification, that solution is hardly a good one.

There is hope, however :)

On Android P the notification in a NotificationChannel of IMPORTANCE_LOW with a priority of PRIORITY_LOW is compacted to a single line even for a ForegroundService. Yeah!!