metrics: Add Cras.NoCodecsFoundAtBoot event to CrosEvents

Also put event names into an array, so it's easier to add new event names.

BUG=none
TEST=run "strace -e write=3 metrics_client -v Cras.NoCodecsFoundAtBoot" and
verify the event is written.

Change-Id: I925b4ff3b6d362e099df62a11905c1df03fc9435
Reviewed-on: https://gerrit.chromium.org/gerrit/46998


Reviewed-by: Chih-Chung Chang <chihchung@chromium.org>
Tested-by: Chih-Chung Chang <chihchung@chromium.org>
Commit-Queue: Chih-Chung Chang <chihchung@chromium.org>