shell: Make singletons owned by ShellGlobal

This means also means they will be cleaned up when disposing
ShellGlobal, which will then mean signals tied to the GObject lifetime
will be disconnected.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6536
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2722>

src/shell-app-usage.c

@@ -763,10 +763,5 @@ on_enable_monitoring_key_changed (GSettings     *settings,
 ShellAppUsage *
 shell_app_usage_get_default (void)
 {
-  static ShellAppUsage *instance;
-
-  if (instance == NULL)
-    instance = g_object_new (SHELL_TYPE_APP_USAGE, NULL);
-
-  return instance;
+  return shell_global_get_app_usage (shell_global_get ());
 }