lmkd: Introduce kill strategy based on zone watermarks, swap and thrashing

Add new kill strategy which makes kill decisions based on which zone
watermark is breached, how much free swap space is still available and
what percentage of the file-backed page cache has been refaulted. This mode
is designed to be used only with PSI signals. It kills unconditionally when
a critical pressure event is received, therefore PSI stall for that event
should be set to a value representing a truly non-responding system
(currently set to 700ms out of 1sec spent in complete stall). New event
handler also controls polling interval based on current memory conditions.

Bug: 132642304
Test: lmkd_unit_test, ACT memory pressure tests
Change-Id: Ia213ef2bb06b245d651ebf2d813e944b4ae7565f
Signed-off-by: Suren Baghdasaryan <surenb@google.com>