Added DispatcherQueue dispatching for high contrast theme change (#4243)

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## Fixes #3253

Applying changes discussed in #3253 (comment).

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## PR Type

What kind of change does this PR introduce?

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- Bugfix
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
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## What is the current behavior?
Changes to the high contrast theme are propagated on the original synchronization context.
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->

## What is the new behavior?
The event handler now dispatches on the current dispatcher queue.
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## PR Checklist

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- [X] Tested code with current [supported SDKs](../#supported)
- [X] New component
  - [X] Pull Request has been submitted to the documentation repository [instructions](../blob/main/Contributing.md#docs). Link: <!-- docs PR link -->
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Author: msftbot[bot]

Microsoft.Toolkit.Uwp.UI/Helpers/ThemeListener.cs

@@ -79,23 +79,26 @@ public ThemeListener(DispatcherQueue dispatcherQueue = null)
             }
         }
 
-        private void Accessible_HighContrastChanged(AccessibilitySettings sender, object args)
+        private async void Accessible_HighContrastChanged(AccessibilitySettings sender, object args)
         {
 #if DEBUG
             System.Diagnostics.Debug.WriteLine("HighContrast Changed");
 #endif
 
-            UpdateProperties();
+            await OnThemePropertyChangedAsync();
         }
 
         // Note: This can get called multiple times during HighContrast switch, do we care?
         private async void Settings_ColorValuesChanged(UISettings sender, object args)
         {
-            await OnColorValuesChanged();
+            await OnThemePropertyChangedAsync();
         }
 
-        // Internal abstraction is used by the Unit Tests
-        internal Task OnColorValuesChanged()
+        /// <summary>
+        /// Dispatches an update for the public properties and the firing of <see cref="ThemeChanged"/> on <see cref="DispatcherQueue"/>.
+        /// </summary>
+        /// <returns>A <see cref="Task"/> that indicates when the dispatching has completed.</returns>
+        internal Task OnThemePropertyChangedAsync()
         {
             // Getting called off thread, so we need to dispatch to request value.
             return DispatcherQueue.EnqueueAsync(

UnitTests/UnitTests.XamlIslands.UWPApp/XamlIslandsTest_ThemeListener_Threading.cs

@@ -39,7 +39,7 @@ public Task Init()
         [TestMethod]
         public async Task ThemeListenerDispatcherTestAsync()
         {
-            await _themeListener.OnColorValuesChanged();
+            await _themeListener.OnThemePropertyChangedAsync();
 
             await _taskCompletionSource.Task;
         }
@@ -49,7 +49,7 @@ public async Task ThemeListenerDispatcherTestFromOtherThreadAsync()
         {
             await Task.Run(async () =>
             {
-                await _themeListener.OnColorValuesChanged();
+                await _themeListener.OnThemePropertyChangedAsync();
             });
             await _taskCompletionSource.Task;
         }