ANDROID: sched/rt: cpu_active_mask intersection

In the event of a partial hotunplug, a stable state with a CPU set in the
online mask but cleared in the active can happen. This is problematic for
the window between the active mask clearing and the sched domains rebuild.
RT could bounce back a task, migrated off a hotunplugged CPU. Introducing
an intersection between lowest_mask and the cpu_active_mask to prevent a
such situation.

Bug: 161210528
Change-Id: I4f8cb782c2ca560c297b7f4bdb2336918c83a5a1
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>