Revert App::run() behavior/Remove winit specific code from `bevy_…

…app` (#10389)

# Objective
The way `bevy_app` works was changed unnecessarily in #9826 whose
changes should have been specific to `bevy_winit`.
I'm somewhat disappointed that happened and we can see in
bevyengine/bevy#10195 that it made things more
complicated.

Even worse, in #10385 it's clear that this breaks the clean abstraction
over another engine someone built with Bevy!

Fixes #10385.

## Solution

- Move the changes made to `bevy_app` in #9826 to `bevy_winit`
- Revert the changes to `ScheduleRunnerPlugin` and the `run_once` runner
in #10195 as they're no longer necessary.

While this code is breaking relative to `0.12.0`, it reverts the
behavior of `bevy_app` back to how it was in `0.11`.
Due to the nature of the breakage relative to `0.11` I hope this will be
considered for `0.12.1`.

src/lib.rs

@@ -349,6 +349,14 @@ impl Default for WinitAppRunnerState {
 /// Overriding the app's [runner](bevy_app::App::runner) while using `WinitPlugin` will bypass the
 /// `EventLoop`.
 pub fn winit_runner(mut app: App) {
+    if app.plugins_state() == PluginsState::Ready {
+        // If we're already ready, we finish up now and advance one frame.
+        // This prevents black frames during the launch transition on iOS.
+        app.finish();
+        app.cleanup();
+        app.update();
+    }
+
     let mut event_loop = app
         .world
         .remove_non_send_resource::<EventLoop<()>>()