code: public static <TResult> Task<TResult> callInBackground(Callable<TResult> callable) {
      return call(callable, BACKGROUND_EXECUTOR, null);
    }
docs: Invokes the callable on a background thread, returning a Task to represent the operation.
  
  If you want to cancel the resulting Task throw a {@link java.util.concurrent.CancellationException}
  from the callable.