settings: implement KeyValueStore (#1659)

* settings: implement KeyValueStore and dependencies

* settings: update KeyValueStore for recent refactoring

* settings: address feedback

libraries/libstratosphere/include/stratosphere/ams/impl/ams_system_thread_definitions.hpp

@@ -114,6 +114,7 @@ namespace ams::impl {
     /* settings. */
     AMS_DEFINE_SYSTEM_THREAD(21, settings, Main);
     AMS_DEFINE_SYSTEM_THREAD(21, settings, IpcServer);
+    AMS_DEFINE_SYSTEM_THREAD(21, settings, LazyWriter);
 
     /* erpt. */
     AMS_DEFINE_SYSTEM_THREAD(21, erpt, Main);