Convert System.Printing-ref that builds on GitHub to a cycle-breaker

On GitHub, System.Printing-ref.csproj builds an assembly that is used pre-dominantly as a cycle-breaker (for internals). The real reference assembly is currently building in dotnet-wpf-int.

We should formally convert this to a true cycle-breaker and trim down the surface area exposed.

This is follow on work from: https://github.com/dotnet/wpf/issues/932 where, in the dotnet-wpf-int portion, we removed the internals from System.Printing's reference assembly. When System.Printing goes open source, this need to be done since the new ref assembly will no longer contain internals needed by the build.