Fix a couple leaks in SystemUI

Changing the navigation mode causes an overlay broadcast which causes all
the fragments in systemui to be recreated.  This could result in leaks
in the status bar icons (lots of bitmaps) and also the sampling callbacks
from SurfaceFlinger.

- Always stop the sampling listener when the mode changes
- Always unregister from device config changes in the QS header

Bug: 141637942
Test: Switch navigation modes a couple times, take hprof dump and verify
      there aren't leaking classes or bitmaps

Change-Id: Id3a28031e2eeef1dc45ed0f4b028771d89e24610