[Design] Rework `ViewDataDictionary` constructors to ensure `ModelMetadata` is ne...

...ver `null`

- `ViewDataDictionary<TModel>` constructors now pass `typeof(TModel)` to base constructors
- remove `ViewDataDictionary<TModel>.ModelMetadata` override
- add more doc comments

nits:
- change `RazorPageActivator` to pass current `ModelStateDictionary`
- change 2 other `ViewDataDictionary` copy constructor calls to avoid
  uselessly copying `Model` and related information from source
- make a few `ViewDataDictionary` properties get-only
I mean "new" as in "different" and will make that change in the comments. basically `object` is the best guess for a declared type because (a) the declared type of the parent `ViewDataDictionary` isn't relevant since we may be looking at a property of that model or something completely different and (b) nothing else tells this constructor a better declared type.