sched: Avoid packing tasks with low sleep time

Low sleep time can be an indication that waking tasks will not receive
any vruntime bonus and hence would suffer from latency when packed.
short-burst tasks sleeping on an average more than sched_short_sleep_ns
are not eligible for packing. This policy covers the case where a
task runs in short bursts and sleeping for smaller duration in between.

Change-Id: Ib81fa37809b85c267949cd433bc6115dd89f100e
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>