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
jewel: ensure /cache is formatted

Change-Id: I80c42fd17fc251309ea148bed45431a1aac4226a