sched: Make use of sysctl_sched_wake_to_idle in select_best_cpu

sysctl_sched_wake_to_idle is a means to allow or disallow a global
task placement preference for idle CPUs. It has been unused thus
far since we've preferred to use a per-task flag instead to control
placement for individual tasks. Using this global flag, however, does
allow greater flexibility for testing and system evaluation.
Incorporate sysctl_sched_wake_to_idle in the placement policy.

Change-Id: I7e830bc914eb9c159ae18f165bc8b0278ec9af40
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>