ANDROID: sched/fair: Make the EAS wake-up prefer-idle aware

Make the mainline EAS wake-up path aware of prefer idle tasks in
preparation for disabling find_best_target().

What is done in the mainline algoritm isn't strictly equivalent to the
find_best_target() algorithm but comes real close, and isn't very
invasive. The main differences with the original find_best_target()
behaviour are the following:

 1. the policy for prefer idle when there isn't a single idle CPU in the
    system is simpler now. We just pick the CPU with the highest spare
    capacity;

 2. the cstate awareness for prefer idle is implemented by minimizing
    the exit latency rather than the idle state index. This is how it is
    done in the slow path (find_idlest_group_cpu()), it doesn't require
    us to keep hooks into CPUIdle, and should actually be better because
    what we want is a CPU that can wake up quickly;

 3. non-prefer-idle tasks just use the standard mainline energy-aware
    wake-up path, which decides the placement using the Energy Model.

Bug: 120440300
Change-Id: I57769c90c57115f6a28d27c5a88e08aa93a30a56
Signed-off-by: Quentin Perret <quentin.perret@arm.com>