[Easy fix] Issue with multiple isolates and FlutterDownloader.registerCallback

Hi,

I work with multiple isolates and each one is scheduled with AlarmManager (https://pub.dev/packages/android_alarm_manager) and each one can start task with FlutterDownloader.
As you known, we must not register multiple ports to an isolate name.

So we have an issue around FlutterDownloader.registerCallback(downloadCallback);
The callback must take an other parameter "isolateNameServer";

Actually this is my callback

```dart
static void downloadCallback(String id, DownloadTaskStatus status, int progress) {
final SendPort send = IsolateNameServer.lookupPortByName('downloader_send_port');
print("Task: $id, status: $status, progress $progress");
send.send([id, status, progress]);
}
```

As the callback is a static we can't change his port name.

Is this little upgrade possible ?