How to disable datepicker UI for a mobile device
To use the native datepicker of the mobile device, the field needs to have type="date". So your code to disable the datepicker popup and use the native device's instead might look like:

(function($) {
  $.ui.datepicker.prototype.options.focus = "button";

  $(document).on('crmLoad', function($region) {
    $('input.dateplugin', $region).each(function() {
      this.type = 'date';
    });
  });
})(CRM.$);