ANDROID: cpufreq/schedutil: Select frequency using util_avg for RT

Schedutil always requests max frequency whenever a RT task is running.
Now that we have a better estimate of the utilization of RT runqueues,
it is possible to make a less conservative decision and scale frequency
according to the needs of the RT tasks.

To do so, protect the RT-go-to-max code with a new sched_feature.
The sched_feature is disabled by default, hence favoring energy savings
as required in mobile environments.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Change-Id: Ic9f01c8703d4f843addaa0d684012a422fe9f3b8