We currently throw if a pending seek position was valid when the user issued
it on the app thread, but can't be resolved on the playback thread because the
timeline changed in the meantime. Throwing in this case seems wrong as the
user could not have known about the issue (and the seek position was actually
valid). Also, in other cases where the currently playing period is no longer
in the new timeline, we gracefully use a subsequent period or transition to
ENDED state instead of throwing. So it seems more consistent to transition to
ENDED state as well.
PiperOrigin-RevId: 236274862
8fce3ce678