ANDROID: sched: Estimate energy impact of scheduling decisions

Adds a generic energy-aware helper function, energy_diff(), that
calculates energy impact of adding, removing, and migrating utilization
in the system.

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Change-Id: I05ce491e5e97c9f30183a4f8f3131c92aa68cecc
Signed-off-by: Chris Redpath <chris.redpath@arm.com>