VolumeDialogImpl: Don't vibrate when volume dialog is not visible

We shouldn't be playing volume dialog specific haptic feedback
when the dialog is not visible.

Test: Call am.setRingerModeInternal(AudioManager.RINGER_MODE_VIBRATE)
      from external application, observe that there's no vibration.
Change-Id: I10ad1e0259092c2297d96f083161395275467781

packages/SystemUI/src/com/android/systemui/volume/VolumeDialogImpl.java

@@ -1578,7 +1578,7 @@ public class VolumeDialogImpl implements VolumeDialog,
 
     protected void onStateChangedH(State state) {
         if (D.BUG) Log.d(TAG, "onStateChangedH() state: " + state.toString());
-        if (mState != null && state != null
+        if (mShowing && mState != null && state != null
                 && mState.ringerModeInternal != -1
                 && mState.ringerModeInternal != state.ringerModeInternal
                 && state.ringerModeInternal == AudioManager.RINGER_MODE_VIBRATE) {