Previously any `CuesWithTiming.durationUs` could be `TIME_UNSET`,
meaning it should be replaced by the next `CuesWithTiming` instance
(instead of being merged if the durations overlap, which is currently
the expected behavior for all `CuesWithTiming` with a 'real' duration).
This technically allowed a single subtitle track to include a mixture of
`CuesWithTiming` that should be merged, and some that should be
replaced. This is not actually needed for any of the subtitle formats
currently supported by ExoPlayer - in all cases a format expects either
all cues to be merged, or each cue to replace the previous one.
Supporting this mixture of merging and replacing in `TextRenderer` ended
up being very complicated, and it seemed a bit pointless since it's not
actually needed. This change means a given subtitle track either merges
**all** cues (meaning `CuesWithTiming.durationUs = C.TIME_UNSET` is not
allowed), or **every** cue is replaced by the next one (meaning
`CuesWithTiming.durationUs` may be set (to allow for cues to 'time out',
needed for CEA-608), or may be `TIME_UNSET`).
This value will be used in a subsequent change that adds cue-merging
support to `TextRenderer`.
PiperOrigin-RevId: 565028066
abaf3e7aa1