Enable 'AcrylicBrush' in WinUI 3 again

[Sergio0694]

PR Type

What kind of change does this PR introduce?

This PR adds AcrylicBrush on WinUI 3 again. On this target, the BackgroundSource property has been removed.

What is the current behavior?

This type is missing for WinUI 3.

What is the new behavior?

The type is usable again.

PR Checklist

Please check if your PR fulfills the following requirements:

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• Tested code with current supported SDKs
• New component
  • Documentation has been added
  • Sample in sample app has been added
  • Analyzers are passing for documentation and samples
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (if applicable)
• Header has been added to all new source files
• Contains NO breaking changes

[michael-hawker]

Future note, as I know this came up elsewhere when we were porting some Media samples, is since the property only exists on WinUI2, it makes it harder to share XAML across both platforms. It'd be nicer if the property existed for WinUI3 even if it was a no-op or always set to one value.

It's a general issue, but something we should be cognizant of.