Make DPC listen to auto-brightness mode changes

- remove brightness mode code from PowerManager

Bug: 161868123
Test: Make sure that switching auto-brightness mode still works correctly.
Test: atest PowerGroupTest
Test: atest PowerManagerServiceTest
Change-Id: Ie8bed709c14a715d2f834eb4404463727f4dab13
Merged-In: Ie8bed709c14a715d2f834eb4404463727f4dab13