Ensure Images handle DPI changes on Windows

The Window Image handler watched for XamlRoot changes so it could react to [RasterizationScale changes](https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.xamlroot.rasterizationscale?view=winrt-22000) and reload the image. But the handling of that event was too aggressive and reloaded the image any time the window size changed.

We need to determine:
- Whether this is something we need to handle at all (does WASDK deal with this for us, or do we need to explicitly reload the image)
- A way to test this (preferably automated, but manual is better than nothing)
- How we can react with a reload _only_ to RasterizationScale changes, rather than _any_ property change on XamlRoot

Original XamlRoot watching code was removed in PR #3231