SysUI:  Prevent Kill-Task from killing SystemUI

If recents screen was active when Kill-Task button pressed, SysUI
was getting killed (causing wallpaper reset).  This prevents SystemUI
from being killed.

Change-Id: Id5b2199605f37bbf5ae0ae6efa0c401ea820ea2d
Merge "SysUI:  Prevent Kill-Task from killing SystemUI" into jb42