Modify Override the Views Filter Criteria Programaticly
You can use hook_form_views_exposed_form_alter() :

/**
 * Implements hook_form_views_exposed_form_alter().
 */
function MY_MODULE_form_views_exposed_form_alter(&amp;$form, &amp;$form_state) {
  if($form['#id'] == 'flag_lists_user_lists_public') {
    $form['usershare']['#value'] = 'admin';
  }
}