Add back backlight brightness curve adjustment.

Change-Id: I58408111d6366a281303afb469d78588ae88d275

Conflicts:
	services/java/com/android/server/power/DisplayPowerController.java
Add method to PowerManager that allows querying the lowest possible
brightness.

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

Change-Id: Idb58fd86dee1a053fe858291be6c9b3169ac4247

Fixups.