getFile VS support dirs on iOS

[g123k]

Hi!

This is a follow-up of the discussion in #6722.

@monsieurtanuki To be understand, please remind me what was the issue with the getFile method initially:

  static Future<File> getFile(String path) async {
    if (Platform.isIOS) {
      final int lastSeparator = path.lastIndexOf('/');
      final String filename = lastSeparator == -1
          ? path
          : path.substring(lastSeparator + 1);
      final Directory directory = await getDirectory();
      path = '${directory.path}/$filename';
    }
    return File(path);
  }

[monsieurtanuki]

@g123k The issue that justified the creation of method getFile is that:

• we use a specific directory (like "application directory") for our temporary pictures in background tasks
• for the record the images are temporary but not to be automatically deleted by the OS, only by us after the server gets the image
• and that folder sometimes evolves on iOS, on the very same device, for some iOS reasons (and does move the files accordingly)
• as we sometimes store the full file path, we had problems because we couldn't find the file anymore as the folder wasn't the same