sched: window-stats: update task demand on tick

A task can execute on a cpu for a long time without being preempted
or migrated. In such case, its demand can become outdated for a long
time. Prevent that from happening by updating demand of currently
running task during scheduler tick.

Change-Id: I321917b4590635c0a612560e3a1baf1e6921e792
CRs-Fixed: 698662
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>