AVRCP: Don't send notifications in callbacks

Since native code can have unbounded wait (for thread safety), only call
it on the main thread.

Some notifications are also idempotent so also eliminate duplicate
notifications if we might send multiple in a short time period.

Test: connect to tesla, observe no crashing
Change-Id: I4ca1ab6d8657ce4f02983101c68e95ebf9026753
Fixes: 36794139