ANDROID: sched/fair: streamline find_best_target heuristics

The find_best_target() code has evolved over time to integrate different
micro-optimizations to the point to be quite difficult now to follow
exactly what it's doing.

This patch rafactors the existing code to make it more readable and easy
to maintain. It does that by properly identifying the three main
use-cases and addressing them in priority order:
 A) latency sensitive tasks
 B) non latency sensitive tasks on IDLE CPUs
 C) non latency sensitive tasks on ACTIVE CPUs

The original behaviors are preserved. Some tests to compare
power/performances before and after this patch have been done using
Jankbench and YouTube and we did not noticed sensible differences.

The only difference with respect of the original code is a small update
to favor lower-capacity idle CPUs in case B. The same preference is not
enforce in case A since this can lead to a selection of a non-reserved
CPU for TOP_APP tasks, which ultimately can lead to non desirable
co-scheduling side-effects.

Change-Id: I871e5d95af89176217e4e239b64d44a420baabe8
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
(removed checkpatch whitespace error)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
[Fixed cherry-pick issue]
(cherry picked from commit 6b0008b7d7be7d141f9855337eb09ad0c9217cfb)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>