AVRCP: don't remove inactive media controllers

Some players change their controllers to inactive when they are
buffering, paused, or changing songs which causes us to lose track of
the controller in these states when it makes sense to keep track still.

In the case where we actually have no controller, simplify the logic
around tracking play state using A2DP feedback.

Test: Pause / Play with troublesome headphones
Bug: 38497029
Change-Id: I8e1258b85007c070fc083d9522f9fa8caa13e6f2