jewel: support variable capacitive button backlight brightness

Requires http://review.cyanogenmod.org/49693 &
Requires http://review.cyanogenmod.org/49614

Moved to device overlay since not all s4 devices support it

Change-Id: Ib20450a77858d50557de8b8663cf448f4aaab17e
Merge "jewel: support variable capacitive button backlight brightness" into cm-10.2