Data traffic details: use proper string for System UID

Android System ≠ Android OS (second is a "kernel" process).
Mind the numbers:
http://goo.gl/YnFvHa (actual "system" process)
http://goo.gl/XwoISO ("system" process displayed as "kernel")
http://goo.gl/sJ9yMi ("kernel" process with different traffic data)

Depends on http://review.cyanogenmod.org/63190

Change-Id: I6f6662b7adaee069ec5f29c6926b8fbce796917d