Save state on suspend on Android and iOS (#5601)

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This pull request fixes a subset of #5492 by saving the application
state when the `suspended` event is received on Android. This way, even
if the user exits the app and closes it manually right after changing
some state, it will be saved since `suspended` gets fired when the app
is exited. It does not fix the `on_exit` function not being fired - this
seems to be a winit bug (the `exiting` function in the winit application
handler trait is not called on exit). Once it gets fixed, it may be
possible to remove logic introduced by this PR (however, I am not sure
how it would handle the app being killed by the system when in the
background, that would have to be tested).

I've tested the logic by:
* Leaving from the app to the home screen, then killing it from the
"recent apps" menu
 * Leaving from the app to the "recent apps" menu and killing it
 * Restarting the device while the app was running

In all of these instances, the state was saved (the last one being a
pleasant surprise). It was tested on the repository mentioned in #5492
with my forked repository as the source for eframe (I unfortunately am
not able to test it in a larger project of mine due to dependence on
"3rd party" egui libraries (like egui_notify) which do not compile along
with the master branch of eframe (different versions of egui), but I
believe it should work in the same manner in all scenarios). Tests were
conducted on a Galaxy Tab S8 running Android 14, One UI 6.1.1.

CI passed on my fork.

* [x] I have followed the instructions in the PR template

crates/eframe/src/native/glow_integration.rs

@@ -366,6 +366,20 @@ impl WinitApp for GlowWinitApp<'_> {
             .and_then(|r| r.glutin.borrow().window_from_viewport.get(&id).copied())
     }
 
+    fn save(&mut self) {
+        log::debug!("WinitApp::save called");
+        if let Some(running) = self.running.as_mut() {
+            profiling::function_scope!();
+
+            // This is used because of the "save on suspend" logic on Android. Once the application is suspended, there is no window associated to it, which was causing panics when `.window().expect()` was used.
+            let window_opt = running.glutin.borrow().window_opt(ViewportId::ROOT);
+
+            running
+                .integration
+                .save(running.app.as_mut(), window_opt.as_deref());
+        }
+    }
+
     fn save_and_destroy(&mut self) {
         if let Some(mut running) = self.running.take() {
             profiling::function_scope!();
@@ -413,7 +427,7 @@ impl WinitApp for GlowWinitApp<'_> {
         if let Some(running) = &mut self.running {
             running.glutin.borrow_mut().on_suspend()?;
         }
-        Ok(EventResult::Wait)
+        Ok(EventResult::Save)
     }
 
     fn device_event(
@@ -1214,10 +1228,12 @@ impl GlutinWindowContext {
             .expect("viewport doesn't exist")
     }
 
+    fn window_opt(&self, viewport_id: ViewportId) -> Option<Arc<Window>> {
+        self.viewport(viewport_id).window.clone()
+    }
+
     fn window(&self, viewport_id: ViewportId) -> Arc<Window> {
-        self.viewport(viewport_id)
-            .window
-            .clone()
+        self.window_opt(viewport_id)
             .expect("winit window doesn't exist")
     }