CRAS: metrics - Init metric handle to let CRAS send events to UMA

We need to call CMetricsLibraryInit function first so that it can
send events successfully.

BUG=chromium:876660
TEST=verify events and samples can be sent to UMA.

Change-Id: I49d0d636d68c83de00292673c3f649fc6222b3df
Reviewed-on: https://chromium-review.googlesource.com/1184806


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Reviewed-by: Hsinyu Chao <hychao@chromium.org>