Use ApplicationDocumentDirectory for Android instead of ExternalStorageDirectory

[Harishwarrior]

The package uses getExternalStorageDirectory which forces the user to add external storage permission in the android manifest. This causes issue with Android 13+ devices which has no permission to access external storage.

// Filing methods:------------------------------------------------------------
  Future<String?> get _localPath async {
    var directory;

    if (Platform.isIOS) {
      directory = await getApplicationDocumentsDirectory();
    } else {
      directory = await getExternalStorageDirectory();
    }

    return directory.path;
  }