[Spare RPH] Add features to tune memory pressure behavior

The first feature allows setting the memory pressure level threshold
that controls when a spare can be created, and when spares are
killed. This is because we've noticed that moderate memory pressure
is very common but is also often transient (~5 seconds). We want
to test if critical memory pressure is a better threshold.

The second feature can be used to control whether only the extra
spares are killed on memory pressure. The rationale for this feature
is that the original behavior was that we did not kill the spare
under memory pressure. This is to retain at least part of the
original behavior when maintaining multiple spare RPHs.

Bug: 364635886
Change-Id: Ica6e79c0230d05cad4d4b72812f6f0d7a07b7bc4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6406919
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1440334}