sched: Track burst length for tasks

Track burst length for tasks as time they ran from wakeup to sleep.
This is used to predict average time a task may run when it wakes up
and thus avoid waking up idle cpu for "short-burst" tasks.

Change-Id: Ie71d3163630fb8aa0db8ee8383768f8748270cf9
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>