Re land of https://github.com/flutter/flutter/pull/142000.
Differences:
1. Fixed the test that was failing in postsubmit. The reason was that the Flutter Gradle Plugin was being applied after KGP in that test, so we couldn't find the KGP version. This caused a log, and the test expects no logs. I moved FGP to after KGP
2. Added to the logs for when we can't find AGP. Change is from
> "Warning: unable to detect project AGP version. Skipping version checking."
to
> ~"Warning: unable to detect project AGP version. Skipping version checking. \nThis may be because you have applied the Flutter Gradle Plugin after AGP."~
update: the above is wrong, changed to
> "Warning: unable to detect project KGP version. Skipping version checking. \nThis may be because you have applied AGP after the Flutter Gradle Plugin."
3. Added a note to the app-level build.gradle templates that FGP must go last
> // The Flutter Gradle Plugin must be applied after the Android and Kotlin Gradle plugin.
Policy per https://flutter.dev/go/android-dependency-versions.
https://github.com/flutter/flutter/issues/140913
~Still a WIP while I clean up some error handling, remove some prints, and figure out a Java test (more difficult than the others because I believe we can only install one java version per ci shard).~
~Also it looks like there are errors that I need to fix when this checking is applied to a project that uses the old way of applying AGP/KGP using the top-level `build.gradle` file (instead of the new template way of applying them in the `settings.gradle` file).~ Done, this is why [these lines exist](9af6bae6b9/packages/flutter_tools/gradle/src/main/groovy/flutter.groovy (L72-L88)) in `flutter.groovy`. They just needed to be added
