lmkd: Detect the highest level of vmpressure when event is detected

(cherry pick from commit e82e15c2)

lmkd checks for vmpressure events using epoll_wait() with eventfds of
all registered events. It's possible that multiple events of different
priorities happen before epoll_wait() returns. For these cases we
use conservative approach by assuming that the system is under the
highest registered vmpressure levels. This speeds up lmkd response time
to high memory pressure by not responding to possibly stale low pressure
levels when vmpressure rises quickly.

Bug: 63631020
Test: alloc-stress

Change-Id: I79a85c3342e7e1b3a3be82945266b2cc60b437cf
Merged-In: I79a85c3342e7e1b3a3be82945266b2cc60b437cf
Signed-off-by: Suren Baghdasaryan <surenb@google.com>