AVRCP: synchronize around mMediaController

There were small windows where mMediaController could be used and
modified by separate threads causing NullPointerExceptions. Synchronize
around mMediaController to avoid these issues.

Test: none
Bug: 62068020
Change-Id: I68acadefe793c4c0f7ffed108ec2e943ade138b8
(cherry picked from commit ad7a50ece55752b9b7e47cd86dc45de368c8c6d5)
1 file changed