Android: Migrate to Flutter Android embedding v2 (fix build on Flutter 3.29+)

[VikasNadcab]

This PR fully migrates flutter_uploader to the Android embedding v2 API (FlutterPlugin) and removes the deprecated PluginRegistry.Registrar, which was removed in Flutter 3.29.0.

Problem

The current 3.0.0-beta.4 version (and older releases) no longer compile on Flutter 3.29+ because of these errors:cannot find symbol import io.flutter.plugin.common.PluginRegistry.Registrar;
public static void registerWith(Registrar registrar)

Fix

• Replaced the old v1 embedding with the modern FlutterPlugin interface
• Implemented onAttachedToEngine / onDetachedFromEngine
• Properly cleaned up all channels, WorkManager observers, and handlers on detach
• Removed dead registerWith code and obsolete imports
• Kept 100% of the existing functionality (progress/result streams, WorkManager integration, etc.)
• Verified the plugin now compiles and runs successfully on Flutter 3.29+ / Dart 3

Impact

Users can finally use flutter_uploader again with the latest stable Flutter without having to downgrade or patch the package locally.

Tested on:

• Flutter 3.29.0 (stable)
• Android API 34
• Physical device + emulator

Closes #285 (and several duplicate issues reporting the same compilation failure)

Thank you for maintaining this package – this should unblock a lot of developers who still rely on it for background uploads!