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
I think by "don't need" tobias means "don't need angular to automatically do this." There are plenty of existing angular apps that have existing a11y strategies, for which this would be extra code weight.