Some classes do not work well on Xaml Islands.

[azchohfi]

Describe the bug

Related to CommunityToolkit/Microsoft.Toolkit.Win32#228

There are a lot of issues when some controls and helpers of the toolkit run in the context of a Xaml Island. We need these fixed.

Steps to Reproduce

For example, try to use the ThemeListener class in a WPF app with Islands. It will throw exceptions.
Anything that leverages the Dispatcher using, for example CoreApplication.MainView.Dispatcher, will not work inside Xaml Islands since there is no such thing as a MainView. We need to refactor those to use the new DispatcherQueue.

Expected behavior

Should work in UWP and inside Xaml Islands (.Net Core).

Environment

NuGet Package(s): 6.0.0

Windows 10 Build Number:

• Fall Creators Update (16299)
• April 2018 Update (17134)
• October 2018 Update (17763)
• May 2019 Update (18362)
• Insider Build (build number: )

App min and target version:

• Fall Creators Update (16299)
• April 2018 Update (17134)
• October 2018 Update (17763)
• May 2019 Update (18362)
• Insider Build (xxxxx)

Device form factor:

• Desktop
• Xbox
• Surface Hub
• IoT

Visual Studio

• 2017 (version: )
• 2019 (version: )
• 2019 Preview (version: )

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