From [ the last change in `DefaultPreloadManagerTest`](2b54b1ebbe), the preloadManager began to use a separate `preloadThread` in `release_returnZeroCount_sourcesAndRendererCapabilitiesListReleased`, which unveils a bug in `PreloadMediaSource`. When `PreloadMediaSource.releasePreloadMediaSource` is called, `preloadHandler` will post a `Runnable` on the preload looper to release the internal resources. Before this `Runnable` is executed, it is possible that the [`stopPreloading`](https://github.com/androidx/media/blob/main/libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/preload/PreloadMediaSource.java#L442) method is executed just as the result of preloading has completed. This is expected to remove the posted `Runnable`s for further preloading, however, the posted `Runnable` for releasing will also be removed from the message queue.
Ideally we should use `postDelayed(runnable, token, delayMillis)` to post the runnables so that the token will be useful to identify which messages to remove in `removeCallbacksAndMessages(token)`, but that `postDelayed` method is only available from API 28. So in this change we are using a separate handler for releasing, and then the call of `preloadHandler.removeCallbacksAndMessages` won't impact the runnable for releasing.
#cherrypick
PiperOrigin-RevId: 696894483
0143884cd7