ANDROID: sched: Move energy-aware decision into a helper function

As we add more conditions to bypass energy-aware wakeups for some tasks,
it will become helpful to have the decision in a self-contained block
of code. Slightly reorganise entrance to rcu read-side lock.

Change-Id: I98a3d2a44204a2c876a59f9d54d392abad4e435a
Signed-off-by: Chris Redpath <chris.redpath@arm.com>