[CrOS MultiDevice] Metrics for MultiDevice.SetupNotification.Dismissed

An enumerated histogram with 3 buckets:
1) “New user, potential host exists”,
2) “Existing user, host switched”, and
3) “Existing user, new Chromebook added”.

Tracks if users are actively dismissing our notifications.

Bug: 870138
Change-Id: If46245c1be7528d63a92d51607c63bded8b391d2
Reviewed-on: https://chromium-review.googlesource.com/c/1316446
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606305}