fix(ngAria): handle non-string model values

Fix for #10212. Compare the string value of the model
so that aria-checked will be properly applied to
type=radio in the case where the model is a number

Closes #10212
suggestions: adda helper function like this:

```js
function ToString(O) {
  return (O == null || !O.toString) ? O : O.toString();
}
```

probably the best bet.