MediaItem.mediaId used to default to the content URI, but this changed:
cc26a92e07
Before the mediaId change linked above, a playlist of different content
all with the same ad URI would play the ads for every item. After the
change the ad would only play once (because mediaId == "" for every
item, so they're all the same). This change restores roughly the
original behaviour by always considering both mediaId and the content
URI.
#minor-release
Issue: #9106
PiperOrigin-RevId: 382763618
