AVRCP: Rework available media player handling

Instead of rebuilding the available player list every time a player
changes, only add and remove the players that have changed.

Add browsable players that don't have an active media session to the
available players.

Update browsable players as they are connected to and refresh the list
(and notify clients) when we are done.

This has an effect of removing a lot of log lines, and reduces churn
of the MediaController callback registration.

Update logging that remains to be much less verbose.

Test: connect to various carkits, verify browsing, updated logs
Bug: 34249902
Bug: 36497125
Bug: 36802046
Change-Id: I9783c682a3f8c22b8b79cfd9023fac6078b21b9f
(cherry picked from commit eef486e7)