shell-app-usage: fix tracking code

We were checking if the app was running, but then marking it as
recently-seen either way. Fix it to only update it when the app is
running.

https://bugzilla.gnome.org/show_bug.cgi?id=640447

src/shell-app-usage.c

@@ -324,6 +324,7 @@ on_app_state_changed (ShellWindowTracker *tracker,
 
   running = shell_app_get_state (app) == SHELL_APP_STATE_RUNNING;
 
+  if (running)
     usage->last_seen = get_time ();
 }