[various] Migrate example apps' AppDelegate.swift (#8155)
Migrated by running:
```
dart run script/tool/bin/flutter_plugin_tools.dart build-examples --ios --swift-package-manager
dart run script/tool/bin/flutter_plugin_tools.dart build-examples --macos --swift-package-manager
```
Tests aren't updated as this change should not affect semantics. I will get a test exemption.
Part of https://github.com/flutter/flutter/issues/159173