s4-common: add support for capacitive button backlight brightness adjustment

Adds slider to "Backlight" option under "Buttons" settings menu
to adjust backlight brightness.

Requires http://review.cyanogenmod.org/49614

Thanks to LorD ClockaN for the original commit on m7-common
(http://review.cyanogenmod.org/49613) on which this commit was based.

Patch Set 3: Remove overlay change from common (will move to device folder)

Change-Id: I4e3bc29cecac54ea156ded613d2027c79a5e9b7e
Merge "s4-common: add support for capacitive button backlight brightness adjustment" into cm-10.2