CmdPal: Remove backdrop targets from existing backdrop controller before assigning a new one (#40540)

## Summary of the Pull Request
Also runs the code through UI thread. it will ensure correct access to
_acrylicController, and also because the method touches UI element.


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## PR Checklist

- [x] **Closes:** #38659 
- [ ] **Communication:** I've discussed this with core contributors
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- [x] **Tests:** Added/updated and all pass
- [x] **Localization:** All end-user-facing strings can be localized
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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/MainWindow.xaml.cs

@@ -107,7 +107,7 @@ public sealed partial class MainWindow : WindowEx,
         App.Current.Services.GetService<SettingsModel>()!.SettingsChanged += SettingsChangedHandler;
 
         // Make sure that we update the acrylic theme when the OS theme changes
-        RootShellPage.ActualThemeChanged += (s, e) => UpdateAcrylic();
+        RootShellPage.ActualThemeChanged += (s, e) => DispatcherQueue.TryEnqueue(UpdateAcrylic);
 
         // Hardcoding event name to avoid bringing in the PowerToys.interop dependency. Event name must match CMDPAL_SHOW_EVENT from shared_constants.h
         NativeEventWaiter.WaitForEventLoop("Local\\PowerToysCmdPal-ShowEvent-62336fcd-8611-4023-9b30-091a6af4cc5a", () =>
@@ -175,6 +175,8 @@ public sealed partial class MainWindow : WindowEx,
 
     private void UpdateAcrylic()
     {
+        _acrylicController?.RemoveAllSystemBackdropTargets();
+
         _acrylicController = GetAcrylicConfig(Content);
 
         // Enable the system backdrop.