[Light Switch] Light Switch should detect changes in Windows Settings and treat as manual override (same as using shortcut) (#42882)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
This PR ensures that Light Switch detects changes to app/system theme
from Windows Settings. This PR also introduces new behavior where
switching the schedule will cause an instant update to the theme if
necessary.

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## PR Checklist

- [x] Closes: #42878
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

src/modules/LightSwitch/LightSwitchService/LightSwitchServiceObserver.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include "SettingsObserver.h"
+
+// The LightSwitchServiceObserver reacts when LightSwitchSettings changes.
+class LightSwitchServiceObserver : public SettingsObserver
+{
+public:
+    explicit LightSwitchServiceObserver(std::unordered_set<SettingId> observedSettings) :
+        SettingsObserver(std::move(observedSettings))
+    {
+    }
+
+    void SettingsUpdate(SettingId id) override;
+    bool WantsToBeNotified(SettingId id) const noexcept override;
+};