Fix 5243349 RemoteControlDisplay incorrectly updated

This fixes a case where the RCD would display transport control
 for a RemoteControlClient that didn't have audio focus.
This was happening because registering an RCD was directly calling
 the updateRemoteControlDisplay method, without first calling
 the checkUpdateRemoteControlDisplay method which verifies the
 conditions before updating the display. One of those conditions
 is that the audio focus stack shouldn't be empty.

To verify this fix, several functions were also rename to clearly
 indicate the lock order and verify we properly synchronize on
 the right objects. In doing so, a missing synchronization on
 audio focus was found.

Change-Id: If1baaac224ea676aeb83ac0aefcc53f87461c32e