CRAS: server_metrics - Log the highest hardware level per device

When we remove device, metrics will log the highest hardware level to UMA. It
can help us to know how often the users get overrun.

BUG=b:116066435
TEST=The highest hardware level can be recorded well on peppy.

Change-Id: I5c42cc33c209b041fc5e88bc1688f50ed4592b13
Reviewed-on: https://chromium-review.googlesource.com/1263699


Commit-Ready: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Tested-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>