Allow getting runtime properties for the active context when it was created through non-context-based entry points

[elinor-fung]

With the introduction of context-based entry points, we will allow getting the runtime properties for the active context used to load the runtime. However, if the active context was created through non-context-based entry points, this is blocked. We should provide a way to do this per dotnet/core-setup#5859 (comment)