sched: Fix load tracking bug to avoid adding phantom task demand

When update_task_ravg() is called with the TASK_UPDATE event on a task
that is not on the runqueue, task demand accounting incorrectly treats
the time delta as execution time. This can happen when a sleeping
task is moved to/from colocation groups. This phantom execution time can
cause unpredictable changes to demand that in turn can result in
incorrect task placement. Fix the issue by adding special handling of
TASK_UPDATE in task demand accounting. CPU busy time accounting already
has all the necessary checks.

Change-Id: Ibb42d83ac353bf2e849055fa3cb5c22e7acd56de
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>