sched: use cumulative window demand for task placement

Use cumulative window demand to estimate CPU frequency at the end of
upcoming WALT window boundary.  Newly waking up task will be placed onto
an already active CPU rather than waking up an idle CPU when by doing so
won't increase current CPU's frequency.

Change-Id: I3bef11fdd05c60995aba11d28c3ea8af925a1d44
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>