This is a direct revert of (the revert of (the reland of (the policy pr))): https://github.com/flutter/flutter/pull/143132.
The only change is:
1. to put a conditional all on one line, because the packages repository has a test that uses an old flutter project to make sure nothing regresses. The old project uses an old gradle version, and the old gradle version bundles an old groovy version, and the old groovy version has a bug where lines that start with `&&` don't always work: https://issues.apache.org/jira/browse/GROOVY-7218 (I enjoy that the revert reason ends up providing another strong justification to go forward with the policy). Also thanks to @reidbaker for pointing out this bug.
2. I also made a slight formatting change to the messages that print when out of the support bounds, which I think looks slightly better.
I tested this with on a branch that included a revert of https://github.com/flutter/flutter/pull/142008, and was able to recreate the failure and verify that it was resolved by 1).
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
