Fix Notifications library for C++ apps (#3423)

* Remove System.ValueTuple

It was seemingly added for no reason with the ToastContentBuilder code: 9c38f71016

Nothing there uses Tuple, so not sure why it was added...

* Revert "Remove System.ValueTuple"

This reverts commit 5ad4fa825e.

* Exclude ValueTuple from C++

Microsoft.Toolkit.Uwp.Notifications/Microsoft.Toolkit.Uwp.Notifications.csproj

@@ -52,7 +52,8 @@
     <None Include="Microsoft.Toolkit.Uwp.Notifications.targets" Pack="true" PackagePath="build\native" />
   </ItemGroup>
 
-  <ItemGroup>
+  <!--Native (C++) doesn't need System.ValueTuple (plus it's incompatible with this package)-->
+  <ItemGroup Condition=" '$(TargetFramework)' != 'native' ">
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
   </ItemGroup>