Fixing CppWinRT_IncludePath and the include paths that build on it to point to the generated headers (#409)

* Fixing CppWinRT_IncludePath and the include paths that build on it to refer to the generated header files rather than the one in the installed Windows SDK.

* Removing redundant condition.

* Resolving issue using alternate approach of setting an include path in props to prevent SDK uap.props from overriding it.

* Remove spacing.

Author: manodasanW

nuget/Microsoft.Windows.CppWinRT.props

@@ -15,6 +15,8 @@ Copyright (C) Microsoft Corporation. All rights reserved.
         <IsNativeLanguage>true</IsNativeLanguage>
         <!-- This causes VS to add the platform WinMD references for the target SDK -->
         <WinMDAssembly>true</WinMDAssembly>
+        <!--Set a value to prevent SDK's uap.props from setting it and pointing to the wrong headers-->
+        <CppWinRT_IncludePath>PreventSdkUapPropsAssignment</CppWinRT_IncludePath>
     </PropertyGroup>
 
     <ItemDefinitionGroup>