sched: change default group upmigrate and downmigrate values

Since the group_[up,down]_migrate_pct tunables are not being used,
update the default threshold values based on 20 ms window sizes
instead of the 10 ms window sizes that the kernel assumes at boot.

Change-Id: If2d08c6be7f69c3e5d6ec589f1452fff8b4c6eee
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>