sched/sched_avg: make number of running tasks conservative

Right now if a task ran 15% in the previous window, that task is
considered as part of number of running tasks as well as while
calculating misfit tasks on smaller capacity clusters which
agressively un-isolates higher capacity CPUs and may lead to
more power consumption. To avoid such issues, make number of
running tasks conservative by changing threshold to 85%.

Change-Id: Ic82724b822bb605fbda385ddeb50a591757d2c9d
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>