Add VmStat atom and read the count of OOM kills

We want to understand how frequently the OOM killer is invoked in our
droidfood / prod fleets. To do so, introduce a /proc/vmstat reading atom
which we can subsequently use as a ValueMetric to count number of OOM
kills per session.

Bug: 154233512
Test: atest VmStatTests
Change-Id: I58ad126aa691c4ee1c61468e4c5ff5331649791c