desktop-capture: honor prefer_cursor_embedded on Wayland portal captures

Before: Wayland PipeWire screen capture always requested cursor metadata, so `prefer_cursor_embedded` was effectively ignored and some users saw no cursor in the video.
After: Wayland PipeWire capture honors `prefer_cursor_embedded` by requesting embedded cursor mode via `xdg-desktop-portal` when opted in (default remains metadata).
Bug: webrtc:469286972
Change-Id: I86a99442984626cc541593a6f3aca236dcb98a75
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/435001
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/main@{#46604}