TTS: Fix a race condition between enqueueSpeechItem and stopForApp.

When speak() is immediately followed by a stop(), the speak may not be
stopped at all due to a race condition.

Bug: 63649835
Test: android.speech.tts.cts.TextToSpeechServiceTest#testSpeakStop
Change-Id: Iae5660b455f5d845aa119c902057e2ca7131ce1b
1 file changed