AudioService: make all BT device updates async

Make AudioManager methods to (dis)connect a BT device
  (hearing aid or A2DP) and handle config update run
  on the AudioDeviceBroker event thread to prevent
  concurrency issues between BtService and AudioService.

Bug: 126239173
Test: have multiple A2DP devices connected, switch between them

Change-Id: Iefd4b4d29e28b15cbb74e8a175ee5e6cc7918c1d