Add tests of visibility of hidden view and change button in system tray

This CL adds the tests of the hidden view and the change button in the
system tray.

This also introduces the Mode::PROHIBITED in AshMessageCenterLockScreen-
Controller, which indicates the lock screen notification is forcibly
prohibited by the policy or the flag.

Bug: none
Test: added unittests pass

Change-Id: I709d295c781848483d84d21058f73a427f0c2bc0
Commit-Queue: Yoshiki Iguchi <>
Auto-Submit: Yoshiki Iguchi <>
Reviewed-by: Tetsui Ohkubo <>
Cr-Commit-Position: refs/heads/master@{#631541}
6 files changed