Add Window::on_present_notify to ack about drawing

That's a way to communicate to winit that you'll present to the window.
While it's a no-op for now, it'll be used to throttle drawing.

src/platform_impl/android/mod.rs

@@ -813,6 +813,8 @@ impl Window {
         self.redraw_requester.request_redraw()
     }
 
+    pub fn pre_present_notify(&self) {}
+
     pub fn inner_position(&self) -> Result<PhysicalPosition<i32>, error::NotSupportedError> {
         Err(error::NotSupportedError::new())
     }