TTS: Ensure to callback onStop() from SynthesisPlaybackQueueItem.

The previous implementation calls back the dispatcher only at the end
of run() loop. It means, if stop() is called after enqueueing and
before running, none of onSuccess, onStop, or onError is called for
the items in the queue.

Bug: 63649835
Test: android.speech.tts.cts.TextToSpeechServiceTest#testSpeakStop
Change-Id: Ib20cb0e9157859866f694439858a27369ba8026e