statsd atom: add WallClockTimeShifted

In ag/4888395 we get rid of wall-clock time from EventMetrics because it
is very RAM-costly. If someone needs the wall-clock time, they can get
it from this atom instead. This way, the wall-clock time is only stored
when interesting, instead of every single event!

Bug: 113072343
Test: manual confirmation that it works
Change-Id: I10c26a4fc9c73b29233108ef1fc0e9797382a8df