Fix camera still being used after closing camera preview page (#1638)

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## Description
<!--- Describe your changes in detail -->
When leaving the camera capture page, the camera now no longer is in
use.

## Motivation and Context
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<!--- If it fixes an open issue, please link to the issue here. -->
Closes #1610
## How Has This Been Tested?
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to -->
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## Screenshots (if appropriate):

## Types of changes
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the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

WinUIGallery/ControlPages/CaptureElementPreviewPage.xaml.cs

@@ -19,6 +19,7 @@
 using Windows.Storage.Streams;
 using System.ComponentModel;
 using WinUIGallery.Helper;
+using System.Threading.Tasks;
 
 namespace WinUIGallery.ControlPages
 {
@@ -38,6 +39,17 @@ public CaptureElementPreviewPage()
             captureContainer.Children.Remove(sv);
             captureContainer.Children.Add(expandToFillContainer);
             expandToFillContainer.Children.Add(sv);
+
+            this.Unloaded += this.CaptureElementPreviewPage_Unloaded;
+        }
+
+        private void CaptureElementPreviewPage_Unloaded(object sender, RoutedEventArgs e)
+        {
+            // Needs to run as task to unblock UI thread
+            if (mediaCapture != null)
+            {
+                new Task(mediaCapture.Dispose).Start();
+            }
         }
 
         private MediaFrameSourceGroup mediaFrameSourceGroup;