metrics daemon: create /var/run/metrics/uma-events earlier

The uma-events file needs to be created earlier, for all users of the
metrics library.  This also avoids permission issues with writing
to /var/run/metrics.

BUG=chromium:376891
TEST=manual
CQ-DEPEND=CL:201416

Change-Id: I3a48a3779edb24f3bd08a91fbee4e8fe5a25ec49
Reviewed-on: https://chromium-review.googlesource.com/201347


Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>