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This CL add two methods in ArcMessageCenterLockScreenController.
- IsEnabled() for visibility of lock screen notification
- GetMode() for mode of lock screen notification

Bug: b/111391270
Test: none

Change-Id: Ieb41c6dcc78768cff12932a62f33153d4f3e172b
Reviewed-on: https://chromium-review.googlesource.com/1164872
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581224}