oomd: threshold swap kill candidates to usages of more than 5%

In some instances, particularly with swap on zram, swap used will be high
while there is still a lot of memory available. FB OOMD handles this by
thresholding kills to X% of total swap usage. Let's do the same thing here.

Anecdotally with these thresholds and my laptop which is exclusively swap
on zram I can sit at 0K / 4G swap free with most of memory free and
systemd-oomd doesn't kill anything.

Partially addresses aggressive kill behavior from
https://bugzilla.redhat.com/show_bug.cgi?id=1941170