OmniGears: cleanup duplicate button brightness setting

Currently, button brightness settings are accessible from 2 locations:

Settings > Display > Brightness

and

Settings > Buttons

Since it's BUTTON settings, let's contain these settings to the BUTTONS category

Change-Id: I4164791d902a750b2ee3c6cf8ebdeddd4162db99

Conflicts:
	res/values-es/custom_strings.xml
OmniGears: FR translation

Change-Id: I7315d175b65374170a69800014d01ae17484a44c