The media item needs to be assigned to `Window.mediaItem` in `CastTimeline.setWindow`. For this the `MediaItem` needs to be available in the timeline.
When a `MediaItem` is passed to the `set/addMediaItems` method, we can't yet know the Cast `MediaQueueItem.itemId` that is generated on the device and arrives with an async update of the `RemoteMediaClient` state. Hence in the `CastTimelineTracker`, we need to store the `MediaItem` by Casts's `MediaItem.contentId`. When we then receive the updated queue, we look the media item up by the content ID to augment the `ItemData` that is available in the `CastTimeline`.
Issue: androidx/media#25
Issue: google/ExoPlayer#8212
#minor-release
PiperOrigin-RevId: 460325235
(cherry picked from commit 30fbc3a2)
