[Fuchsia] Simplify ScenicScreen and FlatlandScreen

In the past ScenicScreen was creating one display instance per window.
That was necessary because device pixel scale wasn't always the same for
all windows. That's no longer the case after crrev.com/889040, so the
class can be simplified: the same display can be shared between all
windows. The same applies to FlatlandScreen.

Change-Id: I50e31fe8ef0184306d0bc89f83a87c8a51b9b1c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3183959
Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#925358}