rq_stats: Doing rq_stats calculation in the scheduler tick.

With this change, we do the average run queue statistics calculation
in the scheduler tick itself. This helps avoid any extra timers to
do the same. Also doing this calculation in the scheduler tick avoids
any bias if the calculation is done in a workqueue

Change-Id: I854d90acc05cc7a7226487be5555976826d8c837
Signed-off-by: Amar Singhal <asinghal@codeaurora.org>