Fix to ContainerView to send array mutation notification after it has set up the childViews array in init()

See #2283.

Because `length` is only ever expired as a consequence of a call to `arrayContentDidChange`, we need to make such a call at the end of `init`, because `length` could be calculated before `childViews` has been set up, and any properties or observers dependent on `length` will break until a subsequent mutation results in a call to `arrayContentDidChange`.