AudioService: more fixes for BT A2DP device switch

Fix a couple of remaining problems left after
commit 4599ee23.

- Increase delay before unmuting media after handling
last message in switch sequence.
- Mute stream immediately instead of via a message.
- Also mute when handling messages for media A2DP
forced usage and route updates because of MediaRouter
interaction while switching device.
- Handle muting before enqueueing the messages in case
the handler thread preempts the sender after the message
is enqueued and before mute is handled.

Bug: 144784716
Test: repro steps in bug.
Change-Id: Iaae8e6d8eb3112e9770085e65afe7fb73119b502