LibCore+Userland: Don't auto-start new Core::Timers

This was unintuitive, and only useful in a few cases. In the majority,
users had to immediately call `stop()`, and several who did want the
timer started would call `start()` on it immediately anyway. Case in
point: There are only two places I had to add a manual `start()`.

Userland/Applications/SystemMonitor/NetworkStatisticsWidget.cpp

@@ -132,6 +132,7 @@ NetworkStatisticsWidget::NetworkStatisticsWidget()
             1000, [this] {
                 update_models();
             });
+        m_update_timer->start();
 
         update_models();
     };