[remoting] Implement curtain-mode for Wayland.

GNOME/Mutter does not expose any API (or even an unsupported method) to
detect if the Wayland session is headless. So this implements
curtain-mode by checking if the GNOME Wayland service is running. If it
is active, it then looks at the command-line to verify it is configured
for headless mode.

Bug: 448421953
Change-Id: Ie464010915da13a454dea9621fc5096a2fb15bb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7001584
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529252}