Fix superfluous onError callbacks.

When stop() is called twice or after done().This relates
to bug 5662598 because users using the old deprecated API
will see two calls to onUtteranceCompleted.

bug:5662598
Change-Id: I5d59cf66b4f4c8650d3f8f9e503ac3f33132c0d0

Author: narayank

core/java/android/speech/tts/AudioPlaybackHandler.java

@@ -428,7 +428,8 @@ private void handleSynthesisDone(MessageParams msg) {
         final AudioTrack audioTrack = params.getAudioTrack();
 
         if (audioTrack == null) {
-            params.getDispatcher().dispatchOnError();
+            // There was already a call to handleSynthesisDone for
+            // this token.
             return;
         }