Partial fix for issue 3515250: video chat and SCO

Do not call directly into AudioSystem in setBluetoothScoOn() but
send a message to the AudioService handler instead. As the
procedure to switch audio path to BT SCO can last some time we should
not block the caller which can run in the UI thread.

Change-Id: I6ac4b5a934d69781db3aebe5d0e8137b52a0ada4