Cache the Binder call for PlaybackState

It's rare but possible that the 2nd Binder call returns a different
value than the 1st one, fix the potential NPE by caching the return
value of MediaController#getPlaybackState

Bug: 283583998
Test: N/A
Change-Id: I9e8f456f3817f68dd98f637a1ee4a7d6d109801e