Fix enqueue, resume and retry callback for Android.

[addie9000]

In Android, it seems that the callback registered by FlutterDownloader.registerCallback method does not work with enqueue, resume and retry. Because sendUpdateProgress does not work due to there is no handler for "updateProgress". I look back the code history and find that before commit 03544c7e0e9e0a0e73b45d66a65cd4b48f79902f it was handle at FlutterDownloader.registerCallback. So I put that code to the same place.

This could fixes #191