Deep links panel displays confusing error message if not supported

[elliette]

Discovered during the monthly DevTools bug bash.

Steps to repro:

1. Connect DevTools to a non-android Flutter app (e.g. a Flutter ios app)
2. There is a dialog that says "You selected a non Flutter project" [screenshot 1] (this is confusing because the connected app is a Flutter app, just not a Flutter android app)
3. Close the dialog
4. Now the deep links tool shows an infinite spinner [screenshot 2], and at the top right, I see "Android variant:" [screenshot 3]

Expected behavior:

The Deep Links panel should detect that a non-Android app is connected, and display a message that the Deep Links tool is only available for Android (ideally with a link to documentation). This is similar to what we do for the Performance panel when connected to a web app [screenshot 4].

FYI @chunhtai @Hangyujin

Screenshots

[1]

[2]

[3]

[4]

[elliette]

Fixed by #9571