Replace references to WinRT with Microsoft.Windows.SDK.NET

[krwq]

Per #1089 (comment) WinRT will not have a builtin runtime support and will cause our library to likely stop working on Windows after 5.0 ships.

Per @jkoritzinsky:

The way to migrate is to remove all references to winmds and System.Runtime.WindowsRuntime and System.Runtime.InteropServices.WindowsRuntime and instead just reference the Microsoft.Windows.SDK.NET package.

[joperezr]

I tried doing this and found a blocker because of issue microsoft/CsWinRT#338 since that Windows SDK package is not currently strong-named signed. Once they fix that issue, we can come back and continue this work

[pgrawehr]

@joperezr : microsoft/CsWinRT#338 is fixed. So you might want to give it another try.