[exo] Trigger output activation notifications from wayland::Server

Currently each instance of WaylandDisplayHandler (client-specific
output handle in exo) maintains its own aura shell observation
so that it can notify clients when the output it corresponds to
has been activated.

This CL moves the observation into exo::wayland::Server code and
delegates to the appropriate WaylandDisplayOutput global wrapper
where appropriate.

This CL concludes the centralization of display-related
observations into the Server class. The result is cleaner and easier
to understand code.

Follow up CLs will extract display-output related logic out of
Server and into an independent class responsible for handling
wayland output objects in exo, making new output extensions easier
to implement and support.

Note: Since output activation is now propagated from Server, the
ActiveDisplay unittest has been moved into the aura shell server
tests. This allows correct widget activation => client output event
testing post refactor.

Bug: 1518335
Change-Id: I3d75d22eb043ac78c094880cb86bbd64b8a9858b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5219895
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1254151}