fix(core): remove once listeners by identity, not stale index - #11306

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What is the current behavior?

Regression introduced in 1e55f0c (#10946), first released in core 9.0.0.

Observable._fireEvent iterates a copy of the observers array but passes the copy's loop index to _handleListenerEntry, which splices the live array at that position. If any listener at a lower index is removed during the same notify pass (a handler calling off() on itself, or a preceding once entry being consumed), the live array shifts left and the splice for a later once entry removes the wrong element or nothing at all. Consequences:

1. The once listener stays subscribed and fires on every subsequent notify.
2. An unrelated listener sitting after it can be silently unsubscribed.

The launch event is affected deterministically in every app: NativeScriptGlobalState registers _setLaunched as the first launch listener and it removes itself inside its handler, so every Application.once('launch', ...) remains permanently armed. On Android this crashes apps that bootstrap from a launch listener and keep their process alive with a foreground service: recreating the activity (e.g. back-button exit, then relaunch) fires launch again, the armed once runs the bootstrap a second time and Application.run() throws Error: Application is already started. inside NativeScriptActivity.onCreate.

Minimal repro:

const obj = new Observable();
obj.on('foo', function handler() {
	obj.off('foo', handler);
});
obj.once('foo', () => console.log('once'));

obj.notify({ eventName: 'foo', object: obj }); // logs "once"
obj.notify({ eventName: 'foo', object: obj }); // logs "once" again