[Keyboard Manager] Fixed app-specific shortcut causing app to lose focus scenario (#4902)

* Fixed focus issue and added tests

* Changed key names

* Use constant instead of hardcoded empty string

src/modules/keyboardmanager/common/KeyboardManagerConstants.h

@@ -96,4 +96,7 @@ namespace KeyboardManagerConstants
 
     // String constant for the default app name in Remap shortcuts
     inline const std::wstring DefaultAppName = L"All Apps";
+    
+    // String constant to represent no activated application in app-specific shortcuts
+    inline const std::wstring NoActivatedApp = L"";
 }