[CompanionPane] update Xamarin.Android project UI to match Kotlin sample

[conceptdev]

This Xamarin Companion Pane example (CompanionPane) is not up-to-date with the latest UX in the Kotlin repo.

It should be possible to bring the resources (images, XML layouts) over to update the Xamarin version to match. README/screenshots should be updated to match.

Xamarin

Kotlin