lmkd: Do not downgrade/ignore events when swap is full

When the swap space is full, a pressure event is unlikely to resolve by
itself.  In this case, do not downgrade or ignore the events.

Bug: 112056451
Test: Fill up swap on a 1GB device and check critical vmpressure events
      are not downgraded.
Change-Id: If154dc364711bf7c86f32e24ddcd10be359386de