getDisplayMode() {
    const displayMode = this.config_.get('displayMode');
    // Make sure the display mode is valid.
    for (let key in UiHandlerConfig.DisplayMode) {
      if (UiHandlerConfig.DisplayMode[key] === displayMode) {
        // Return valid flow.
        return UiHandlerConfig.DisplayMode[key];
      }
    }
    // Default to option first mode.
    return UiHandlerConfig.DisplayMode.OPTION_FIRST;
  }
Returns the display mode for the tenant selction flow. Default to
option first mode.
@return {!UiHandlerConfig.DisplayMode} The display mode.