Move AudioService recv off of main thread

Some of the calls in the AudioService broadcast receiver are expensive,
and posting them to the main thread can lead to slow-down/ANR.
Additionally, there is no need to handle these events on the main
thread.

Move the receiver to a dedicated handler thread. Eventually, this should
be combined with the handler we already have for events.

Test: Smoke test (bt connect)
Fixes: 335457435
Change-Id: Ibc6b95f7bd628a63c565adfa7f5d52cdcda9fb1b