Capture idle callback before running it (#54984)

Summary:
Fixes https://github.com/facebook/react-native/issues/54983

https://github.com/facebook/react-native/issues/54983 has more details, but in short, this code here:

```kotlin
currentIdleCallbackRunnable?.cancel()
currentIdleCallbackRunnable = IdleCallbackRunnable(frameTimeNanos)
reactApplicationContext.runOnJSQueueThread(currentIdleCallbackRunnable)
```

If `currentIdleCallbackRunnable` change between line 2 and line 3 (which is known to happen), the wrong runnable would be sent to be run. The fix is to simply capture it in a local variable before passing it to be run

```kotlin
currentIdleCallbackRunnable?.cancel()
val idleCallbackRunnable = IdleCallbackRunnable(frameTimeNanos)
currentIdleCallbackRunnable = idleCallbackRunnable
reactApplicationContext.runOnJSQueueThread(idleCallbackRunnable)
```

## Changelog:

[ANDROID] [FIXED] - Capture idle callback before running it

Pull Request resolved: https://github.com/facebook/react-native/pull/54984

Test Plan:
Unfortunately this is a pretty speculative change. That said, since the change is so simple, the worst thing that can happen is nothing. I have also run it in production here with no known issues
https://github.com/bluesky-social/social-app/pull/9436

Reviewed By: cortinico

Differential Revision: D90120471

Pulled By: lunaleaps

fbshipit-source-id: c853a976be999fa883a5835fd5e8ac75a8699681