[remoting] Host protocol support for ActiveDisplay message.

This updates the proto with the website's definition for the
ActiveDisplay message. Some code is added to support sending the
new client message from the host.

Bug: 1447942
Change-Id: I996c6bd5b6ab4fa800568ba549035be74072c396
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4553486


Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147596}