Fix tts tests for Android Wear.

All test from android.speech.tts.cts.TextToSpeechTest contained checks
for mTts == null except one: testSpeakStop and it was failing on Android
Wear, so I added the check to that test as well.

Bug: 30901940
Change-Id: I924bb30a1b65a05c9b0ae7499d91b37db3302f9b
diff --git a/tests/tests/speech/src/android/speech/tts/cts/TextToSpeechTest.java b/tests/tests/speech/src/android/speech/tts/cts/TextToSpeechTest.java
index 43b0fe3..c117745 100644
--- a/tests/tests/speech/src/android/speech/tts/cts/TextToSpeechTest.java
+++ b/tests/tests/speech/src/android/speech/tts/cts/TextToSpeechTest.java
@@ -144,6 +144,9 @@
     }
 
     public void testSpeakStop() throws Exception {
+        if (mTts == null) {
+            return;
+        }
         getTts().stop();
         final int iterations = 20;
         for (int i = 0; i < iterations; i++) {