Refactor locks in "audio" service for device management

Do not use shared locks across AudioDeviceBroker, BtHelper
  and AudioDeviceInventory.
Synchronize package private  tmethods on BtHelper and avoid
  dealing with external locks.
Clean up code ordering, more nullability annotations and
  annotation for connection state.

Bug: 112863932
Test: regression test on BT usecases
Change-Id: Id9e19e3c5e5241af913d06275f325fe505c1b509