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

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## 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.

<!-- Please review the items on the PR checklist before submitting-->
## 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

Author: Jaylyn-Barbee

src/modules/LightSwitch/LightSwitchService/LightSwitchService.cpp

@@ -74,6 +74,7 @@
   </ItemDefinitionGroup>
   <ItemGroup>
     <ClCompile Include="LightSwitchService.cpp" />
+    <ClCompile Include="LightSwitchServiceObserver.cpp" />
     <ClCompile Include="LightSwitchSettings.cpp" />
     <ClCompile Include="SettingsConstants.cpp" />
     <ClCompile Include="ThemeHelper.cpp" />
@@ -84,6 +85,7 @@
     <ResourceCompile Include="LightSwitchService.rc" />
   </ItemGroup>
   <ItemGroup>
+    <ClInclude Include="LightSwitchServiceObserver.h" />
     <ClInclude Include="LightSwitchSettings.h" />
     <ClInclude Include="SettingsConstants.h" />
     <ClInclude Include="SettingsObserver.h" />

src/modules/LightSwitch/LightSwitchService/LightSwitchService.vcxproj

@@ -33,6 +33,9 @@
     <ClCompile Include="WinHookEventIDs.cpp">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="LightSwitchServiceObserver.cpp">
+      <Filter>Source Files</Filter>
+    </ClCompile>
   </ItemGroup>
   <ItemGroup>
     <ClInclude Include="ThemeScheduler.h">
@@ -53,6 +56,9 @@
     <ClInclude Include="WinHookEventIDs.h">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="LightSwitchServiceObserver.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <Natvis Include="$(MSBuildThisFileDirectory)..\..\natvis\wil.natvis" />

src/modules/LightSwitch/LightSwitchService/LightSwitchService.vcxproj.filters

@@ -0,0 +1,29 @@
+#include "LightSwitchServiceObserver.h"
+#include <logger.h>
+#include "LightSwitchSettings.h"
+
+// These are defined elsewhere in your service module (ServiceWorkerThread.cpp)
+extern int g_lastUpdatedDay;
+void ApplyThemeNow();
+
+void LightSwitchServiceObserver::SettingsUpdate(SettingId id)
+{
+    Logger::info(L"[LightSwitchService] Setting changed: {}", static_cast<int>(id));
+    g_lastUpdatedDay = -1;
+    ApplyThemeNow();
+}
+
+bool LightSwitchServiceObserver::WantsToBeNotified(SettingId id) const noexcept
+{
+    switch (id)
+    {
+    case SettingId::LightTime:
+    case SettingId::DarkTime:
+    case SettingId::ScheduleMode:
+    case SettingId::Sunrise_Offset:
+    case SettingId::Sunset_Offset:
+        return true;
+    default:
+        return false;
+    }
+}

src/modules/LightSwitch/LightSwitchService/LightSwitchServiceObserver.cpp

@@ -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;
+};

src/modules/LightSwitch/LightSwitchService/LightSwitchServiceObserver.h

@@ -39,6 +39,7 @@ void LightSwitchSettings::InitFileWatcher()
             GetSettingsFileName(),
             [this]() {
                 Logger::info(L"[LightSwitchSettings] Settings file changed, signaling event.");
+                LoadSettings();
                 SetEvent(m_settingsChangedEvent);
             });
     }
@@ -65,6 +66,11 @@ void LightSwitchSettings::NotifyObservers(SettingId id) const
     }
 }
 
+HANDLE LightSwitchSettings::GetSettingsChangedEvent() const
+{
+    return m_settingsChangedEvent;
+}
+
 void LightSwitchSettings::LoadSettings()
 {
     try

src/modules/LightSwitch/LightSwitchService/LightSwitchSettings.cpp

@@ -79,7 +79,7 @@ class LightSwitchSettings
 
     void LoadSettings();
 
-    HANDLE GetSettingsChangedEvent() const { return m_settingsChangedEvent; }
+    HANDLE GetSettingsChangedEvent() const;
 
 private:
     LightSwitchSettings();

src/modules/LightSwitch/LightSwitchService/LightSwitchSettings.h

@@ -2,6 +2,7 @@
 
 #include <unordered_set>
 #include "SettingsConstants.h"
+#include "LightSwitchSettings.h"
 
 class LightSwitchSettings;
 
@@ -22,7 +23,7 @@ class SettingsObserver
     // Override this in your class to respond to updates
     virtual void SettingsUpdate(SettingId type) {}
 
-    bool WantsToBeNotified(SettingId type) const noexcept
+    virtual bool WantsToBeNotified(SettingId type) const noexcept
     {
         return m_observedSettings.contains(type);
     }