AVRCP: synchronize with better granularity

Make synchronization more granular by synchronizing on the objects
actually being used.

Fix missing synchronization on mMediaPlayerInfoList.

Test: basic smoke test with multiple players
Change-Id: Iec451dca2c51ec6f6f35a1d0a28f2c4dd52db722
Fixes: 37651901
(cherry picked from commit a303cd6e430b887aa2c74fe585a323617d1837ab)