Avrcp: Limit available players changed

According to the spec, we should only send an Available Players Changed
update when a player is added or removed. (AVRCP 1.6.1 Sec 6.9.4 p74)

Only send an update when a player is added or removed.  If a player
would change feature bits, display name, major or minor type, present it
as a new player instead and remove the old player.

Sync available players notifications with other notifications.

Test: switch players, note that Available Players Changed is not sent
all the time
Change-Id: Icd3730afa6e182810920f28fa7db17b98e53ceea