APContainer: fix AP not showing after screen off

When playing a video with "Play in background" enabled, if the screen is switched off, the player will switch to audio, triggering a ACTION_SHOW_PLAYER scheduled with a 100L delay that has been there since java.

Because the screen is switched off, onPause is called, triggering a cancellation of ACTION_SHOW_PLAYER in the LifecyleAwareScheduler, before it can execute it.

To ensure ACTION_SHOW_PLAYER is executed when switching the screen back on, a flag is set and checked in onResume to call showAudioPlayerImpl