Request From Flutter Team: Migrate to `registrar.viewController` API

This plugin currently relies on legacy UIApplication APIs to access the root view controller ([code here](https://github.com/fluttercommunity/flutter_downloader/blob/4b28cbec41b2c51882d68522420b0787a03e8887/ios/Classes/FlutterDownloaderPlugin.m#L1150)). With Apple's mandatory adoption of the UIScene lifecycle in iOS 27, these legacy patterns will result in unexpected behavior. Please migrate to the new `registrar.viewController` API to ensure compatibility.

For detailed instructions, please see [the official UIScene adoption guide for plugins](https://docs.flutter.dev/release/breaking-changes/uiscenedelegate#migrate-a-flutter-plugin).

Consider using the provided [SKILL](https://github.com/flutter/flutter/issues/190156#issuecomment-5108050627) designed to identify and refactor these legacy patterns.

Start in ios/Classes/FlutterDownloaderPlugin.m around line 1150, where the plugin accesses the root view controller through legacy UIApplication APIs. Read Flutter's UIScene adoption guide for plugins and migrate this lookup to registrar.viewController; done means the plugin no longer relies on the legacy pattern and remains compatible with the UIScene lifecycle.

Written by the indexing model from the issue text.