Fix incomplete consolidation of kill-all-background

It is accidental missed in commit aaef7319, the method
killPackageProcessesLocked with the most parameters is
able to be reused for kill-all-background.

Bug: 114117787
Test: Launch some activities and put them in background,
      Enter command "adb shell am kill-all", logcat should
      print some logs as "kill all background".

Change-Id: I2a5700a36843d369bc5ee165bf6fa4405ea87e87