sched: Use only partial wait time as task demand

The scheduler currently either considers a tasks entire wait time as
task demand or completely ignores wait time based on the tunable
sched_account_wait_time. Both approaches have their limitations,
however. The former artificially boosts tasks demand when it may not
actually be justified. With the latter, the scheduler runs the risk
of never being able to recognize true load (consider two CPU hogs on
a single little CPU). To achieve a compromise between these two
extremes, change the load tracking algorithm to only consider part of
a tasks wait time as its demand. The portion of wait time accounted
as demand is determined by each tasks percent load, i.e. a task that
waits for 10ms and has 60 % task load, only 6 ms of the wait will
contribute to task demand. This approach is more fair as the scheduler
now tries to determine how much of its wait time would a task actually
have been using the CPU if it had been executing. It ensures that tasks
with high demand continue to see most of the benefits of accounting
wait time as busy time, however, lower demand tasks don't experience a
disproportionately high boost to demand triggering unjustified big CPU
usage. Note that this new approach is only applicable to wait time
being considered as task demand and not wait time considered as CPU
busy time.

To achieve the above effect, ensure that anytime a task is waiting, its
runtime in every relevant window segment is appropriately adjusted using
its pct load.

Change-Id: I6a698d6cb1adeca49113c3499029b422daf7871f
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>