ANDROID: sched: get_nohz_timer_target() active_mask filtering

In the event of an incomplete _cpu_down, where cpu_active_mask !=
cpu_online_mask, a !active CPU could be selected by this function to be
the target of a timer. It is likely to happen when this function is called
from a !active CPU. (i.e. cpu_active(smp_processor_id()) == False). In
this scenario, the CPU is probably !idle and even if it would, as !active,
the CPU isn't part of any sched_domains and we can't proceed with the
regular !idle CPU search.

This is a problem, as we want to maximize the idle time for the !active
CPU and hence we prefer to exclude, as much as possible, !active CPUs.

Bug: 161210528
Change-Id: I1fddee7cd746c6739a00f95f8e60453bd44991b0
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>