[Breaking Change] Android io.flutter.facade being removed

[xs...@google.com]

If you haven’t developed Flutter in an add-to-app fashion, integrating a Flutter module into an existing Android app, you can stop reading. 

If you have previously followed our experimental wiki to add Flutter to your existing Android app via https://github.com/flutter/flutter/wiki/Add-Flutter-to-existing-apps and have used the experimental classes io.flutter.facade.Flutter or io.flutter.facade.FlutterFragment, those classes are now deprecated and removed in #44369 in favor of our new Android APIs at io.flutter.embedding.

See the updated guide at https://github.com/flutter/flutter/wiki/Adding-Flutter-to-Android for instructions on using the new Java classes. 

When you `flutter upgrade` past https://github.com/flutter/flutter/pull/44369 on the master channel, the io.flutter.facade.* classes will no longer be available. To keep using those classes until you migrate to the new io.flutter.embedding classes, run `flutter make-host-app-editable` in your Flutter module before `flutter upgrade`ing and source control the android/ directory which contains the Java files for those io.flutter.facade classes. 

Once you’ve migrated to the io.flutter.embedding classes, you can simply delete the android/ directory in your Flutter module.

Please contact us on https://github.com/flutter/flutter/issues/34458 if you run into issues while performing this conversion.