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: I3b669b29e5c244246ac02ad1ce6b4e39f7270aaa
Signed-off-by: Zaphod <mwilson@acesappraisal.com>