The source can be used in compositions (in fact, every source is
automatically used in an internal composition when constructing the
playlist), and there is not really a concept of top-level media source
any more since the Player supports playlists.
The actual restriction is that the content media source needs to have
exactly one period to be able to create a SinglePeriodAdTimeline.
#minor-release
PiperOrigin-RevId: 357544191
