code: public void createToken(@NonNull final Card card, @NonNull final TokenCallback callback) {
          createToken(card, mDefaultPublishableKey, callback);
      }
docs: The simplest way to create a token, using a {@link Card} and {@link TokenCallback}. This
  runs on the default {@link Executor} and with the
  currently set {@link #mDefaultPublishableKey}.
  
  @param card the {@link Card} used to create this payment token
  @param callback a {@link TokenCallback} to receive either the token or an error