Add PowerManager integration for button and keyboard backlight.

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

Change-Id: I550cccafc0a8f90d6347de9261adb26b75955cc4
Merge "Add PowerManager integration for button and keyboard backlight." into cm-10.2
Merge branch 'cm-10.2' of git://github.com/CyanogenMod/android_frameworks_base into cm-10.2