Add additional stats for swapping from /proc/vmstat

This adds swap-in and swap-out rate calculation on the same intervals
as major page fault collection.

BUG=chromium:261965
TEST=observe Platform.SwapIn{Long,Short} and
Platform.SwapOut{Long,Short} appear in histograms

Change-Id: Ifcdba1088cdff355b8e132145ac79635b185663b
Reviewed-on: https://gerrit.chromium.org/gerrit/64162


Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Sonny Rao <sonnyrao@chromium.org>
Tested-by: Sonny Rao <sonnyrao@chromium.org>