android: exit event loop on MainEvent::Destroy

[jihaohaaaa]

When an Android Activity is destroyed, Android dispatches an onDestroy lifecycle event. In the underlying glue layer (such as android-activity), this translates into MainEvent::Destroy.

Previously, winit only logged a TODO warning on MainEvent::Destroy without exiting the event loop. In environments where the host Activity or glue layer expects the native event loop to finish upon destruction (such as GameActivity's android_app_free(), which synchronously waits on pthread_cond_wait for the native thread to exit), this prevented run_app() from terminating and led to a deadlock in onDestroy.

This PR calls self.window_target.exit() and marks self.running = false on MainEvent::Destroy, allowing the event loop to exit cleanly and native resources to be dropped.

Supersedes #4697.