lacros: Add GetDisplayedNotifications to MessageCenter crosapi

This method returns a list of the ids of displayed notifications. Later
CLs will use this for two things:

- To re-synchronize lacros-chrome's cached view of existing
  notifications after a crash (see C++ NotificationPlatformBridge
  GetDisplayed() method).
- To integration-test the MessageCenter crosapi.

Bug: 1134389
Test: added to unit_tests
Change-Id: Iad877a20216c813b72c265d1b23deb1cfeed1353
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445849
Reviewed-by: Jun Mukai <mukai@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813243}