Listen for playback state changes

PlaybackState changes, such as seeking, may not trigger a MediaData
update event from the MediaDataManager. So, attach a callback to the
MediaController to receive the PlaybackState change after seeking..

Fixes: 156950762
Test: atest tests/src/com/android/systemui/media/SeekBarViewModelTest.kt
Change-Id: I16e0d0804715c982e79ab596e7632af3f87c6bd7