Add method to PowerManager that allows querying the lowest possible
brightness.

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

Change-Id: Idb58fd86dee1a053fe858291be6c9b3169ac4247
Add back backlight brightness curve adjustment.

Change-Id: I58408111d6366a281303afb469d78588ae88d275

Conflicts:
	core/java/android/provider/Settings.java
	packages/SystemUI/src/com/android/systemui/statusbar/powerwidget/BrightnessButton.java
	services/java/com/android/server/power/DisplayPowerController.java