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