ANDROID: sched/fair: add idle state filter to prefer_idle case

The CPU selection process for a prefer_idle task either minimizes or
maximizes the CPU capacity for idle CPUs depending on the task being
boosted or not.

Given that we are iterating through all CPUs, additionally filter the
choice by preferring a CPU in a more shallow idle state. This will
provide both a faster wake-up for the task and higher energy efficiency,
by allowing CPUs in deeper idle states to remain idle.

Change-Id: Ic55f727a0c551adc0af8e6ee03de6a41337a571b
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>