function getServiceNames(propertyStore) {
  var props = propertyStore.getProperties();
  return Object.keys(props).filter(function(key) {
    var parts = key.split('.');
    return key.indexOf(STORAGE_PREFIX_) == 0 && parts.length > 1 && parts[1];
  }).map(function(key) {
    return key.split('.')[1];
  }).reduce(function(result, key) {
    if (result.indexOf(key) < 0) {
      result.push(key);
    }
    return result;
  }, []);
}
Gets the list of services with tokens stored in the given property store.
This is useful if you connect to the same API with multiple accounts and
need to keep track of them. If no stored tokens are found this will return
an empty array.
@param {PropertiesService.Properties} propertyStore The properties to check.
@return {Array.<string>} The service names.