sched: inherit the group id from the group leader

When sysctl_sched_enable_thread_grouping is set to 1, any new tasks
created are put in the same group as their group leader.

CRs-Fixed: 984456
Change-Id: If1837dd7c8120c8b097cfffa1dc52eb4781f1641
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>