If I should start a foreground service, startForeground(id,notification) method needs a Notification. I am using PlayerNotificationManager.createWithNotificationChannel to add a notification to player, but I can't use this notification in startForeground (it's not public). Shall I display 2 notifications?!
You do need a foreground service. The notification alone is not sufficient.
PlayerNotificationManager has callbacks that it invokes when it starts and stops showing a notification. You can use these to put your service into (and out of) the foreground. There's an example of this here.