ANDROID: sched/fair: return idle CPU immediately for prefer_idle

When a CPU is selected for a prefer_idle task through find_best_target
this CPU will become a target CPU on which we perform an energy diff and
select between the previous CPU and the target CPU based on the estimated
energy consumption for both placements.

For prefer_idle tasks we should favour performance over energy savings
and therefore return a found idle CPU immediately.

Change-Id: I2242a51134ac172dd58b3b08375388a7b4b84af0
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Suggested-by: Joel Fernandes <joelaf@google.com>