Remove references to deprecated v1 Android embedding

Remove references to deprecated v1 Android embedding

To ensure this plugin will work with users running the next stable version of Flutter remove references to the v1 embedding.

The v1 embedding was deprecated around 6 and a half years ago. In Flutter 3.22, the Flutter tool dropped support for building v1 apps entirely. Flutter plans to delete version one of it's Android embedding in the next stable release, per https://medium.com/flutter/whats-new-in-flutter-3-22-fbde6c164fe3 (in the "Removal of v1 Android embedding" section).

When that deletion happens, any apps using plugins that still have references to the removed classes will fail to build when updating to the next stable version (probably Flutter `3.26`).

As a part of this deprecation, the Flutter Android team is doing community outreach by filing bugs and submitting some pull requests to update plugins to minimize the impact this removal has on the community.

#### Some example PRs:
- Flutter team maintained plugins: https://github.com/flutter/packages/pull/6494
- `android_alarm_manager_plus` https://github.com/fluttercommunity/plus_plugins/pull/2864

#### Consequences:
- This change should have no impact on users of this plugin that are building apps using the v2 embedding (we don't have a percentage to share, but this is the vast majority of users).
- It will remove support for users that are still using the v1 embedding on versions of Flutter older than `3.22`.

If this change doesn't land, this plugin will instead be broken for all apps building on the next stable version of Flutter and beyond.

#### Is there a way to support both v1 and v2 embedding apps in the next stable version of Flutter?
No.

This is the PR where we are removing the deprecated engine classes: https://github.com/flutter/engine/pull/52022

Look at the Android embedding code in the plugin, likely under android/src/main/kotlin or java. Remove any references to deprecated v1 embedding classes like FlutterActivity, PluginRegistry, or old embedding lifecycle methods. Check the example PRs linked in the issue for guidance. Test by building the example app to ensure it still works with Flutter 3.22+.

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