Mash cleanup: message center part 2.

Move ShowLockScreenNotificationSettings, which has little to do with the
rest of the AshMessageCenterController/Client, to LoginScreenClient.

TBR=tsepez@chromium.org

Bug: 958191
Change-Id: I211a1c89d8dc15b2315132152ea32d27770aeb7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1650955
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668165}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/mock_login_screen_client.h[diff]
• ash/public/cpp/login_screen_client.h[diff]
• ash/public/interfaces/ash_message_center_controller.mojom[diff]
• ash/system/message_center/message_center_controller.cc[diff]
• ash/system/message_center/message_center_controller.h[diff]
• ash/system/message_center/notifier_settings_view_unittest.cc[diff]
• ash/system/unified/notification_hidden_view.cc[diff]
• chrome/browser/notifications/chrome_ash_message_center_client.cc[diff]
• chrome/browser/notifications/chrome_ash_message_center_client.h[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]