Create PromosManagerCoordinator

A PromosManagerCoordinator should be created which (1) creates a
PromosManagerMediator, (2) adds PromosManagerMediator as an observer of
PromosManagerSceneAgent, and (3) handles PromosManagerCommands.

Change-Id: Ie8e5fef56d5c2bf29d520dac1c375a36589f374a
Bug: 1358990
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3862972


Commit-Queue: Benjamin Williams <bwwilliams@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1042257}