Don't call StopRecognition when recognition was aborted.

If recognition has been aborted, then the call to the sound trigger
device stop recognition has already occurred. The sound trigger helper
would then try to stop it again, which generates an error code for a
stop without a corresponding start event.

BUG=24677430

Change-Id: Ibf5d1da1a8eb06b677e428f047905d15fd5cf21f