feat(ngAria): add an ngAria module to make a11y easier

I made some significant refactors to #8342, notably:

* DRYing up the implementation
* unDRYing up the specs so that the assertions/failures are more specific
* Naming watch functions to make debugging easier
Should be removed from this service, as it is only used by `ngModel`. Add `$$watchNgModelProperty` instead to the `$aria` service.