sound trigger: fix cross deadlock between service and client

Do not call callback methods on ISoundTriggerClient with the service mutex
locked: the client implementation will lock its mutex to process
the callback and if a request from the same client with client
mutex locked was waiting for the service mutex a deadlock occurs.

Bug: 16873049.
Change-Id: Ifc3b31fdfaed525981fa59876c041478f38b42e8