Errorprone: SynchronizeOnNonFinalField

Synchronizing on non-final fields is not safe: if the field is ever
updated, different threads may end up locking on different objects.

Test: m Bluetooth
Bug: 344658662
Flag: Exempt, enforce error in the futur
Change-Id: I7c15a137d691cf17701635e8b9392eefaae82d81