Deprecated version

During the running I see the error in console:

> The plugin `flutter_webview_plugin` uses a deprecated version of the Android embedding.
> To avoid unexpected runtime failures, or future build failures, try to see if this plugin supports the Android V2 embedding. Otherwise, consider removing it since a future release of Flutter will remove these deprecated APIs.
> If you are plugin author, take a look at the docs for migrating the plugin to the V2 embedding: https://flutter.dev/go/android-plugin-migration.
Please, upgrade the plugin

## System info

Issue occurs on: Android
Plugin version: 0.4.0
Flutter doctor output:

```
Flutter (Channel stable, 2.8.0, on macOS 11.6.1 20G224 darwin-x64, locale en-PL)
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
[✓] Xcode - develop for iOS and macOS (Xcode 13.1)
[✓] Chrome - develop for the web
[✓] Android Studio (version 4.1)
[✓] IntelliJ IDEA Ultimate Edition (version 2020.3)
[✓] VS Code (version 1.63.0)
[✓] Connected device (2 available)```

## Steps to Reproduce

1. Just run any app with Flutter version 2.8.0. Android 30.0.3

リンクされているFlutter Android plugin migration guideから始め、Flutter 2.8.0でアプリをAndroid上で実行して警告を再現します。プラグインのAndroid実装を確認し、V2 embeddingへ移行します。アプリがdeprecated-embedding warningなしで実行されれば完了です。

索引モデルが issue の本文から書いたものです。