sched: Track average sleep time

Similar to tracking average burst length for tasks, average sleep time
indicates how much a task sleeps on an average before waking up to run.
Very low sleep and burst lengths indicates tasks that could be
sensitive to task-wake latencies and hence should not be packed.

Change-Id: Ife68a9a9a9e596246aab5029f60e41c5bad781e4
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>