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
Lots of things don't have `toString` methods --- `Object.create(null)`, `{ toString: null }`, `null`, to name a few. Safer bet is just `'' + newVal`