proc: relax write permissions of sched_group_id file

Related tasks are grouped together based on their sched_group_id.
The userspace implementation requires world write permissions to
this /proc/<PID>/sched_group_id file.

CRs-Fixed: 984456
Change-Id: I92e6fd3ca693387aeb0664cc75afde06f75d4a9f
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>