Add PowerManager integration for button and keyboard backlight.

Allows setting button and keyboard backlight brightness as well as
button timeout.

Change-Id: I550cccafc0a8f90d6347de9261adb26b75955cc4

Conflicts:
	core/java/android/os/PowerManager.java
	core/java/android/provider/Settings.java
	services/java/com/android/server/power/PowerManagerService.java

(cherry picked from commit f0c5766d1747f4adfe4364bb0611765e1171c9a2)