[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
Making the ctors protected sounds like a good idea. It would prevent either our code or user code accidentally setting up these types in ways we don't anticipate and impossible to fix. Not super sure what the benefit of the `Clone` method is - would user code call into this?