devicesettings: button light notification switch

Taking advantage of blinking buttons kernel
patch that enables switching on/off of the periodic
blinking of button leds synchronized with the blinking of the
notification leds (amber/green). This patch
adds a new setting to the Device settings called
Button Light Notification.

Needs kernel change: http://review.cyanogenmod.org/37891

Change-Id: I0f3aae944be25f25d30ba1da5e438b45e8fba03e