Packaging changes

[manodasanW]

• Publish cswinrtgen, cswinrtimpl, cswinrtprojectiongen to AOT during the build and package them in the NuGet package
• Generalized process by adding IsBuildTool property which indicates this is a tool to publish to AOT during build
• Package the task also in the nuget package
• props / target changes to get it working with the .NET SDK and also as part of the cswinrt package itself
• Updated notice file
• Cleanup based on cswinrtgen targets is included by the cswinrt nuget package targets