ANDROID: sched/fair: Factor out CPU selection from find_energy_efficient_cpu

find_energy_efficient_cpu() is composed of two steps; we first look for
the CPU with the max spare capacity in each frequency domain, and then
the impact on energy of each candidate is estimated. In order to make it
easier to implement other CPU selection policies, let's factor the
candidate selection algorithm out of find_energy_efficient_cpu(), and
mark the candidates in a mask. This should result in no functional
difference.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Change-Id: I85a28880f01fcd11d7af28f9fbc1fe0cf4f197cf