Remake kb/button backlight controls as a PowerManager client

PM now has observers for these settings values and applies them in
realtime, so piggyback on those for button light management

Change-Id: I7568069bd6cacf5cbb56c8d0a1746894725b87fa

Conflicts:
	src/com/android/settings/cyanogenmod/BootReceiver.java
	src/com/carbon/settings/fragments/ButtonBacklightBrightness.java
	src/com/carbon/settings/fragments/KeyboardBacklightBrightness.java