AVRCP: Check mMediaSessionManager is not null before handling callback

While the Bluetooth is turning off and MediaPlayerList is cleaned up,
handling the unexpected onAddressedPlayerChanged() callback from the
MediaSessionManager causes the NPE. This NPE happens at
getActivePlayer().unregisterCallback() in the setActivePlayer() function
due to the return value of getActivePlayer() will be null.

Before handling the onAddressedPlayerChanged() callback, add a check of
mMediaSessionManager is not cleaned up to fix this issue.

Bug: 135727092
Test: make, tune the timing of cleaning up MediaPlayerList to hit this
issue, perform Airplane mode on/off and AVRCP related tests.

Change-Id: I7ef66d1977a6e31c989e2d1ec4a87ca8144b3778