AudioService: mute music during BT A2DP device switch

Mute music stream when processing Bluetooth A2DP
device connection or disconnection messages instead of
relying on Bluetooth A2DP service to do it.
This fixes issues due to the async nature of device
disconnection and connection handling.

Bug: 144784716
Test: repro steps in bug.
Change-Id: I83b66cf35b39c29e82f9a6199cd6d31346258038