sched: avoid stale cumulative_runnable_avg HMP statistics

When a new window starts for a task and the task is on a rq, scheduler
decreases rq's cumulative_runnable_avg momentarily, re-account task's
demand and increases rq's cumulative_runnable_avg with newly accounted
task's demand.  Therefore there is short time period that rq's
cumulative_runnable_avg is less than what it's supposed to be.
Meanwhile, there is chance that other CPU is in search of best CPU to place
a task and makes suboptimal decision with momentarily stale
cumulative_runnable_avg.

Fix such issue by adding or subtracting of delta between task's old
and new demand instead of decrementing and incrementing of entire task's
load.

Change-Id: I3c9329961e6f96e269fa13359e7d1c39c4973ff2
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>