Bring the baby back: longpress back button to kill the app [2/2]

this old CM baby (made by @rascarlo) is still useful
till kufi does his magic on the navbar :D

Patchset2: use the new L SwitchPref slider instead of the old
checkbox

Change-Id: Id45b1d28b6864e6bcdfc3c26b4d4137f6369fb38