Update S.T.Json sources with ifdefs so that a netstandard compatible source package...

Update S.T.Json sources with ifdefs so that a netstandard compatible source package can be produced (dotnet/corefx#34611)

* Test changes necessary to build S.T.Json sources targetting netstandard

* Revert unnecessary change from private to internal.

* Revert props and csproj changes to support netstandard along with test
changes.

* Address PR feedback.

* Revert "Revert props and csproj changes to support netstandard along with test"

This reverts commit dotnet/corefx@7a2f22e127c4ef0476f20bddd64796045f79ddb5.

* Fix code formatting/spacing.

* Invert the ifdef so it is false by default (for source package) and add
comments.

Commit migrated from https://github.com/dotnet/corefx/commit/36893daefdc114306efe86eca4170836bc02ff68