Use _states and _state throughout ember-views.

Resolves #4764

At this point, we do not have a good way to make `view.state === 'inDom'` style checks to work. I started with a computed property, but since the general usage assumes that `.state` is a standard property the computed route will not allow us to be backwards compatible.

I can definitely make both the old and new properties work, but I don't see how I would make a deprecation if the old way was used.

Anyone have a suggestion?