Safely clean up the cover art manager

This patch changes 3 things:
(1) Cover art is assumed false until read true
(2) We no longer clean up the CA manager if the feature isn't
enabled. This will keep us from having NPEs since an enabled feature on
start always translates to an object. Plus, since its defaulted to off,
this makes calling stop() before start() safe for cover art.
(3) The stop and start functions are now synchronized so there are no
race conditions inviting rare errors.

Bug: b/154132155
Test: Build, flash, interop test with AVRCP target device
Change-Id: I5a3733160424bd71a40ceb85d685e2e5e8414f99
Merged-In: I5a3733160424bd71a40ceb85d685e2e5e8414f99