[Display Cutout] Add UKM metrics

Add a Layout.DisplayCutout.StateChanged UKM event that
is recorded when display cutout state changes.

BUG=825890

Change-Id: I7b134117ea847505e62f3690d0ac45ab02db8e1f
Reviewed-on: https://chromium-review.googlesource.com/1117638
Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575717}
8 files changed