metrics: move sample file from /var/run to /var/lib

Some metrics are generated near shutdown and we expect to collect
them at reboot, but /var/run is a tmpfs and /var/run/uma-events
is lost.

This changes the uma events file location from the perspective
of the metrics daemon.  I am making the same change to Chrome.
I am adding a symlink to allow an older Chrome to read the
samples from the old location.  A newer Chrome will be screwed
but I hope that's OK (i.e. this will work for official builds
and for Chrome OS developers, but may cause malfunction for
Chrome developers)

BUG=chromium:447256
TEST=verified that /var/lib/metrics/uma-events grows and is truncated
CQ-DEPEND=CL:258971

Change-Id: I677cda16486de239dd205247083d4ad7240d992b
Reviewed-on: https://chromium-review.googlesource.com/257084


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