Make startRecognition async

- This is needed for telephony and audio integration which should happen via async callbacks
that'll end up starting/stopping recognition.

e.g. if a startRecognition happens when in a phone call - the onDetectionStarted will get called once the phone
call ends.

For now the transient stoppages due to internal reasons will not be propagated back to the client.

Bug: 16514535
Bug: 16515468
Change-Id: I1b2b8edd28f5c5e67c453f66c23e1a67a626114e