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/Libraries/LibCore/Timer.cpp

@@ -17,8 +17,8 @@ Timer::Timer(Object* parent)
 Timer::Timer(int interval_ms, Function<void()>&& timeout_handler, Object* parent)
     : Object(parent)
     , on_timeout(move(timeout_handler))
+    , m_interval_ms(interval_ms)
 {
-    start(interval_ms);
 }
 
 void Timer::start()