Refine single monitor view

In the case where the user has only a single monitor, currently we show a list of monitors with only a single thing in it, and make the user click on it to select it and click "Share"

This is a subpar UX, and makes the user do unnecessary work to comprehend what's going on and click on the single list item.

Instead, let's show a simplified view for this use case, and just require that the user click the "Share" button.

Single monitor view, before	Single monitor view, after

cc @apol @grulich @teams/usability @teams/vdg

(VDG and usability folks, let's try to avoid redesigning this whole dialog as that is a separate thing that will be handled soon)