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