selectCallback = function(resp, opt_error) {
    if (resp && resp['account'] && opt_onAccountSelected) {
      opt_onAccountSelected(
          firebaseui.auth.Account.fromPlainObject(resp['account']));
    } else if (opt_onAddAccount) {
      // Check if accountchooser.com is available and pass to add account.
      var isUnavailable = firebaseui.auth.acClient.isUnavailable_(opt_error);
      // Either an error happened or user clicked the add account button.
      opt_onAddAccount(!isUnavailable);
    }
  }
Initializes the accountchooser.com API object. Only the first call
initializes the client and subsequent calls return immediately.

@param {function()=} opt_onEmptyResponse The callback function that is
    invoked when there is no response from accountchooser.com.
@param {function(firebaseui.auth.Account)=} opt_onAccountSelected The
    callback function invoked when an account is selected from
    accountchooser.com.
@param {function(boolean)=} opt_onAddAccount The callback function invoked
    when add account button is clicked in accountchooser.com. A boolean
    availability flag is passed. It is true if accountchooser.com is
    available, false otherwise.
@param {Array<string>=} opt_providers The accepted IdP list.
@param {?string=} opt_language The display language for accountchooser.com.
@param {Object=} opt_uiConfig The UI configuration for accountchooser.com.