lmkd: add ability to monitor all vmpressure events

Ability to monitor all available vmpressure event levels is needed
to accommodate systems with different memory resources. Low memory
systems can rely on medium and critical level events because working
under memory pressure is usual mode of operation. High performance
systems with more memory need to react earlier using also low
vmpressure level events to free memory early and prevent low memory
condition affecting its performance.

Bug: 63631020
Test: alloc-stress

Change-Id: I0cef1bd4c97d32c005045ae47f0ce3464ed98899
Signed-off-by: Suren Baghdasaryan <surenb@google.com>