ANDROID: sched/fair: unify spare capacity calculation

Given that we have a few sites where the spare capacity of a CPU is
calculated as the difference between the original capacity of the CPU
and its computed new utilization, let's unify the calculation and use
that value tracked with a local spare_cap variable.

Change-Id: I78daece7543f78d4f74edbee5e9ceb62908af507
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>