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

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

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/run.rs

@@ -89,6 +89,7 @@ impl<T: WinitApp> WinitAppWrapper<T> {
         event_result: Result<EventResult>,
     ) {
         let mut exit = false;
+        let mut save = false;
 
         log::trace!("event_result: {event_result:?}");
 
@@ -126,6 +127,10 @@ impl<T: WinitApp> WinitAppWrapper<T> {
                     );
                     Ok(event_result)
                 }
+                EventResult::Save => {
+                    save = true;
+                    Ok(event_result)
+                }
                 EventResult::Exit => {
                     exit = true;
                     Ok(event_result)
@@ -139,6 +144,11 @@ impl<T: WinitApp> WinitAppWrapper<T> {
             self.return_result = Err(err);
         };
 
+        if save {
+            log::debug!("Received an EventResult::Save - saving app state");
+            self.winit_app.save();
+        }
+
         if exit {
             if self.run_and_return {
                 log::debug!("Asking to exit event loop…");