Unity/Application: Improve Waydroid window and launcher matching

Fixup for the thing that does the droids:

The window's proper appID is already attached, just not where QtMir was looking for it. Fix that and search for related surfaces by appID before falling back to session-based lookups.

This improves the Waydroid support immensely, resulting in properly matched windows.

In addition, upstream agreed to have some sort of watch mechanism in place that keeps the launcher process running until all windows have been closed. This allows Waydroid to fit nicely into our existing app lifecycle.