Add mutex-based protection mechanism to the A2DP JNI layer

The protection mechanism is needed to avoid a race condition during
graceful cleaning up of the Bluetooth A2DP Interface when Bluetooth
is disabled, and native upcalls like connection_state_callback()

Bug: 62116636
Test: manual
Change-Id: I8324ac0c041d69ce98f39268b376565cfbb5edf5