ANDROID: sched/core: use TG's shares to bias tasks placement

As a simple task group based heuristic, let's consider latency sensitive
tasks which have a bigger then standard shares quota. The idea is that,
if for a TG we wanna allocate more CPU time it's also likely it's more
imporant then others.

Change-Id: I96c7c763725586e30169fcb1db0249c6d9018b73
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>