Add externs for module angular-local-storage

For info about angular-local-storage, see https://github.com/grevory/angular-local-storage
I know we don't use that in other externs but the best way to type a namespace is to use

```js
/**
 * @const
 */
var angularLocalStorage = {};
```

See https://github.com/google/closure-compiler/wiki/A-word-about-the-type-Object#namespaces.