Fix ambiguous TTS#playSilence method.

Due to recent fixes for TTS API review (b/17389935)
overloaded playSilence method became ambiguous when
the last argument is null. This change renames the new
method to playSilentUtterance solving the issue.

Also, since LMP API level is 21, I updated invalid
references to it as API level 20 inside speech.tts.

Bug: 17389935
Change-Id: I9f9fae988240d1cec45d44650e53ffc29803ccac