[System.Net.Http] Move Mac/iOS specific HttpMessageHandler's to Xamarin.*.dll (#13746)
* Revert "[System.Net.Http] Move NSUrlSessionHandler from xamarin-macios to mono (#13229)"
This reverts commit
5bf74a995465dd54ceb4a2311c2d6041258e8fe0.
* [System.Net.Http] Move Mac/iOS specific HttpMessageHandler's to Xamarin.*.dll
We decided to move these types into Xamarin products since they can't be
used without the underlying infrastructure anyway.
* Bump API snapshot submodule
* [csproj] Update project files