async openPicker() {
      this._isPickerOpen = true;
      this._calendarCurrentPicker = "day";
      this.responsivePopover = await this._respPopover();
      this.responsivePopover.showAt(this);
    }
Opens the picker.
@public
@async
@method
@name sap.ui.webc.main.DatePicker#openPicker
@returns {Promise} Resolves when the picker is open