Fix 5485925: Fix crash in transport control view

This fixes a crash where we weren't hiding the view when there's
no registered media player.  It now hides the view when AudioManager
tells it to clear the data.

It also avoids a possible race condition where a request can
come in from AudioManager before the callback is set.

Change-Id: I3360e775c75396e1eee11413d317df5b39ceb487