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
}
```