The MockPlayer currently:
- uses separate fields for single mediaItem vs multiple mediaItems
- replaces all items on addMediaItem operations
- does nothing on remove/move operations.
Fix all of this by using a single field that replicates the player
operations directly.
Some tests also need to be updated to make them more realistic
(for example only removing items from a playlist that have
previously been added).
PiperOrigin-RevId: 451857271
342be88d81