Audit steps to create an add-to-app module

[helin24]

The steps here seem broken: https://github.com/flutter/flutter-intellij/blob/main/docs/testing.md#add-to-app-module-integration-test

I don't see an option to add a Flutter module to an Android application according to these instructions. I'm also not sure if there is a parallel iOS add-to-app workflow similar to this.