libcutils: fix system-background tasks shown in top command

tasks in system-background cgroup will be shown as below:
 580   580 system   12  -8   1% S 190656K  19408K unk surfaceflinger  /system/bin/surfaceflinger

This patch fix this issue.

Change-Id: Ib39ff892010498548d7fbfb4021df62498acd033
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>