For context #84305

I'm planning on adding a warning to the Flutter tool if an app is using the old Android v1 embedding, and mark the classes as deprecated in the engine.

The use of deprecated APIs may produce build failures with the default linter settings, although users can disable the linter or whitelist the references they still have in the code to deprecated APIs.

As a result, it's necessary to remove references from some repos:

• https://github.com/flutter/flutter/search?q=io.flutter.app.FlutterApplication
• https://github.com/flutter/plugins/search?q=io.flutter.app.FlutterApplication

cc @stuartmorgan