ANDROID: sched/fair: Add energy_diff dead-zone margin

It is not worth the overhead to migrate tasks for tiny insignificant
energy savings. To prevent this, an energy margin is introduced in
energy_diff() which effectively adds a dead-zone that rounds tiny energy
differences to zero. Since no scale is enforced for energy model data
the margin can't be absolute. Instead it is defined as +/-1.56% energy
saving compared to the current total estimated energy consumption.

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Change-Id: I89c0043fcc414f5e57eb6264b76b6137cc43c886
Signed-off-by: Chris Redpath <chris.redpath@arm.com>