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 remote-tracking branch 'cm/cm-10.2' into cm-10.2