Add nullable annotations to System.Runtime (dotnet/corefx#37855)

Add nullable annotations to System.Runtime (dotnet/corefx#37855)

* Add nullable annotations to System.Runtime

* Update to use Nullable property with new compiler

* Fix annotations from API Review

* Address nullable feedback from API Review

* Fix more nullable annotations based on review

* More attributes from review and annotate Uri members

* Enable covariant and contravariant nullable annotation mismatch warnings globally

* Fix build

Commit migrated from https://github.com/dotnet/corefx/commit/aa3a6f2b169a82c44007d84aec57050abd5eeb07