metrics library: move event file location from /var/log to /var/run

The event file, which is used for communicating metric events
from chrome os to chrome, has been living in the very non-kosher
location /var/log/metrics/uma-events.  This change duplicates it
in /var/run/metrics/uma-events.  A later chrome change will pick
up the metrics from there.

I also opened crbug.com/373833 to remove the duplication once
Chrome has switched over.

BUG=chromium:361331
TEST=tested with unit tests and manually

Change-Id: I98b741798aa0481b7987f93f087239ad214b759f
Reviewed-on: https://chromium-review.googlesource.com/200091


Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>