Finish volume panel after launching sound settings

Otherwise the activity behind volume panel will be moved to
front first. The causes the animation of activity launch to
show the reorder in a short time, which looks flickering.

Fix: 330105146
Test atest BottomBarViewModelTest
Test: On home screen. Click volume key.
      Press menu to launch VolumePanelActivity.
      Press Settings to launch SoundSettingsActivity.
      Home should not cover on VolumePanelActivity in transition.

Change-Id: I6e5fd97581cee47bf64ce11158169693b6c30236