sched: Honor schedtune.prefer_idle flag

The schedtune.prefer_idle flag indicates that task is latency
sensitive. Place the task on an idle CPU. If none of the CPUs
are idle, select the least loaded CPU. This flag does not affect
cluster selection.

Change-Id: I9b10dbcf2d50bfa2b703f8807c33380d5d5a256d
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>