code: public static <D> Async<D> async(final Callable<D> task) {
      return async(null, task);
    }
docs: Create an asynchronous operation that executes a provided task creating
  an Android's {@link android.os.AsyncTask}.
  @param task job to do
  @param <D> result data type
  @return implementation of {@link Async}, never {@code null}