Add a flag for multiple triggers with same recognition session

Also annotate the flags with @IntDef to make things clearer and safer

Add more debug logging

Revert to start/stop being synchronous since telephony and microphone will
need to be handled internally.

Bug: 16731586
Bug: 16514535
Bug: 16549061
Change-Id: I83695d52e9547269c95d443e4d921c9238b7401e