VolumeControlService: Push registerCallback and unregisterCallback to the handler thread

Resolves two issues:
- registerCallback invokes an aconfig flag which _can_
  require READ_DEVICE_CONFIG permission, this prevents
  the caller implicitely requiring the permission
- mCallbacks is not otherwise protected against
  concurrent access from the handler thread

Test: atest CtsBluetoothTestCases
Bug: 307408418
Change-Id: I4fd8e84e44bac8f272bbfff28377473d689a9979