eSpeakActivity.java: fix the 'X has leaked a ServiceConnection' error.

The TTS engine should be destroyed in the onStop method, not the
onDestroy method to prevent it being used in the stopped state.

android/src/com/reecedunn/espeak/eSpeakActivity.java

@@ -79,8 +79,8 @@ public class eSpeakActivity extends Activity {
     }
 
     @Override
-    public void onDestroy() {
-        super.onDestroy();
+    public void onStop() {
+        super.onStop();
 
         if (mTts != null) {
             mTts.shutdown();