AVRCP: Prevent multithread issues in cleanup

When cleaning up the object in doQuit it was possible to have multiple
threads updating at once, causing NullPointerExceptions in some cases.

Synchronize the startup and shutdown of the Avrcp object to prevent
this.

Test: toggle bluetooth rapidly
Bug: 62676895
Change-Id: I2618c7d7b21acb1f95cb67366d469da2fa88d4a4