Fix regressions in TTS completion callbacks.

(a) onUtteranceCompleted should be called on errors too. Also,
fix up the error handling so that onUtteranceCompleted is
always called.
(b) Don't treat empty utterances as errors, and let the engine
synthesize them, as before.

bug:5662598
Change-Id: I9223592bc6fe5f47d71103f4f02f046b54a655a8