There's no issue for this PR. I can create one if requested.

## Summary

This PR makes public fields of `FlutterExtension` non-static. The aim is to make migrating from Gradle Groovy DSL to Gradle Kotlin DSL easier for Flutter developers, because...

### Without this PR

**android/app/build.gradle.kts**

```kotlin
plugins {
    id "com.android.application"
    id "dev.flutter.flutter-gradle-plugin"
}

android {
    namespace = "io.flutter.examples.hello_world"
    compileSdk = FlutterExtension.compileSdkVersion

    defaultConfig {
        applicationId = "io.flutter.examples.hello_world"
        minSdk = FlutterExtension.minSdkVersion
        targetSdk = FlutterExtension.targetSdkVersion
        // ...
    }
}
// ...
```

Groovy and Java allow accessing static fields of a class through its instance, but Kotlin is being more "correct" and disallows that.

### With this PR

Thanks to this PR, the user won't have to replace `flutter` with FlutterExtension in some places, thus decreasing possible confusion.

```kotlin
plugins {
    id "com.android.application"
    id "dev.flutter.flutter-gradle-plugin"
}

android {
    namespace = "io.flutter.examples.hello_world"
    compileSdk = flutter.compileSdkVersion

    defaultConfig {
        applicationId = "io.flutter.examples.hello_world"
        minSdk = flutter.minSdkVersion
        targetSdk = flutter.targetSdkVersion
        // ...
    }
}
// ...
```