ANDROID: cpu/hotplug: protect _cpu_down against partial HP

In the event of a partial _cpu_down, (i.e. _cpu_down(target) where
target > CPUHP_AP_OFFLINE), the cpu_online_mask won't be aligned with
cpu_active_mask. This is an issue when trying to offline the last CPU
from cpu_active_mask, while num_online_cpus() > 1.

Protect against this case by checking num_active_cpus() instead of
num_online_cpus().

Bug: 161210528
Change-Id: Ibe7d9ef69e5f91e99be0d98076614a7654bda094
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>