ANDROID: sched/tune: Introducing a new schedtune attribute prefer_idle

Hint to enable biasing of tasks towards idle cpus, even when a given
task is negatively boosted. The mechanism allows upto 20% reduction in
camera power without hurting performance.

bug: 28312446
Change-Id: I97ea5671aa1e6bcb165408b41e17bc82e41c2c9e
Signed-off-by: Andres Oportus <andresoportus@google.com>