Added screen brightness override feedback.

Added code to reflect the screen brightness override in the brightness
slider, and reset any user changes to it to communicate that they're
not taking effect.

Test: manual (launch an app that sets the screen brightness override
      and see that it's reflected in the brightness slider, and that
      any user changes to it are reset).

Change-Id: Id6758e6fdfeb80a6c206d08215c1001a437a23c3
Fixes: 78006624