Entire pr generated with [ktlint](https://github.com/pinterest/ktlint) --format. First step before enabling linting as part of presubmit for kotlin changes.

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).

Reverts flutter/flutter#143132

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.

Reverts flutter/flutter#142000

Initiated by: vashworth

Reason for reverting: Causing `Mac_pixel_7pro run_release_test` and `Mac_arm64_android run_debug_test_android` to fail: https://ci.chromium.org/ui/p/flutter/builders/prod/Mac_pixel_7pro%20run_release_test/539/overview
https://ci.chromium.org/ui/p/flutter/builders/prod/Mac_arm64_android%20run_debug_test_android/6682/overview

Original PR Author: gmackall

Reviewed By: {reidbaker, camsim99, bartekpacia}

This change reverts the following previous change:
Original Description:
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](https://github.com/flutter/flutter/blob/9af6bae6b9a8d7a8a363467b834f52bfc64c9336/packages/flutter_tools/gradle/src/main/groovy/flutter.groovy#L72-L88) in `flutter.groovy`. They just needed to be added

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](https://github.com/flutter/flutter/blob/9af6bae6b9a8d7a8a363467b834f52bfc64c9336/packages/flutter_tools/gradle/src/main/groovy/flutter.groovy#L72-L88) in `flutter.groovy`. They just needed to be added

This PR increases Android's `minSdkVersion` to 21.

There are two changes in this PR aside from simply increasing the number
from 19 to 21 everywhere.

First, tests using `flutter_gallery` fail without updating the
lockfiles. The changes in the PR are the results of running
`dev/tools/bin/generate_gradle_lockfiles.dart` on that app.

Second, from
[here](https://developer.android.com/build/multidex#mdex-pre-l):
> if your minSdkVersion is 21 or higher, multidex is enabled by default
and you don't need the multidex library.

As a result, the `multidex` option everywhere is obsolete. This PR
removes all logic and tests related to that option that I could find.
`Google testing` and `customer_tests` pass on this PR, so it seems like
this won't be too breaking if it is at all. If needed I'll give this
some time to bake in the framework before landing the flutter/engine
PRs.

Context: https://github.com/flutter/flutter/issues/138117,
https://github.com/flutter/flutter/issues/141277, b/319373605

Refactor Flutter Gradle Plugin so it can be applied using the declarative `plugins {}` block (#123511)

This PR aims to resolve #121552.

Resources used:
- [Developing Plugins](https://docs.gradle.org/current/userguide/custom_plugins.html)
- [Using Gradle Plugins](https://docs.gradle.org/current/userguide/plugins.html#sec:plugins_block)
- [Composite Builds Plugin Development Sample](https://docs.gradle.org/current/samples/sample_composite_builds_plugin_development.html)

This PR also paves way for #121541, because apps will no longer have:

```groovy
apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
```

hardcoded. Instead, they'll use:

```groovy
plugins {
    // ...
    id "dev.flutter.flutter-gradle-plugin" // the exact name is tentative
}
```