Add tutorial on sharing data between Android/iOS and embedded Flutter

[eseidel]

Is there an existing issue for this?

• [X] I have searched the existing issues
• [X] I have read the guide to filing a bug

Use case

Users often come to us (Shorebird) and previously me in my role as Flutter director, confused as to how to share data between their add-to-app Flutter and their existing app.

Proposal

There appears to be an old example on this, "books": https://github.com/flutter/samples/tree/master_archived/add_to_app

But I don't see something similar in the modern examples: https://github.com/flutter/samples/

There is a whole guide on adding a little Android to your Flutter: https://docs.flutter.dev/platform-integration/platform-channels

But there is no mention of going the other way. Even in the add-to-app tutorial: The closest we get is: https://docs.flutter.dev/add-to-app/android/plugin-setup

The basic question is "I have some data in my Android app, I'm adding a Flutter screen, how do I share data with that screen". Given that all of the top most used Flutter apps are add-to-app (or were last I knew) this feels worth documenting. :)

Then again, given the presence of the (archived) "books" sample, maybe I'm just looking in the wrong place?

FYI @RedBrogdon

Thank you!