The serialization scheme used here is custom, it doesn't need
to be compatible with emsg-v0 or emsg-v1 (since
https://github.com/google/ExoPlayer/commit/97183ef55866170807910cd626264d82d41d46d4).
This means that C.TIME_UNSET will propagate correctly through the
serialization.
#minor-release
Issue: #9123
PiperOrigin-RevId: 382762873
