Revert "Make Quiet Hours tile refresh upon state change."

This reverts commit bc08d4afa9f2258a74f60fa1169866ebf18876f1.
Add method to PowerManager that allows querying the lowest possible
brightness.

Used by the auto-brightness adjustment to adjust the sliders.

Change-Id: Idb58fd86dee1a053fe858291be6c9b3169ac4247