AVRCP: Fix concurrency issues with MediaController

Some callbacks could null the MediaController when we are updating it -
only update in one place and synchronize.

Test: change players while player removes
Bug: 38228028
Change-Id: I4c5f7e9d51cddabeb7f3e48da58f89202f7362e4
(cherry picked from commit f6521ba818df22d8a853d25f7c6adc1e724875e6)
1 file changed