Convert Flutter projects into real Gradle projects

The experimental feature that enables Grade sync for the Android module is too fragile. It keeps breaking in ways that are hard to fix (see #5399), and relies on unstable APIs. We need to remove it.

We should be able to add a couple Gradle files to the top-level project that cause it to appear to be a Gradle project that includes the Android module. That would enable full support for editing Java and Kotlin code. We need to be careful to not inadvertently break the Flutter usage of Gradle. We should be able to re-use some of the add-to-app code. (Adding a Flutter module to an Android project automatically converts the module into a Gradle project.)