sched: window-stats: Account wait time

Extend window-based task load accounting mechanism to include
wait-time as part of task demand. A subsequent patch will make this
feature configurable at runtime.

Change-Id: I8e79337c30a19921d5c5527a79ac0133b385f8a9
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>