topshim: a2dp - Pass selectable capabilities to client

 - Implement missing a2dp disconnected state callback.
 - Pass selectable capabilities to audio server through
  OnBluetoothAudioDeviceAdded callback.
 - Change A2dpCodecIndex enum from i32

Bug: 189497374
Tag: #floss
Test: Manual test with audio server changes
Change-Id: If132c570275def19d37f70d2522d51505f1d4aee