SubtitleExtractor is using IndexSeekMap with only one position to
indicate that its output is seekable. SubtitleExtractor is keeping Cues
in memory anyway so more seek points are not needed. SubtitleExtractor
gets notified about seek occurrence through seek() method. Inside that
method extractor saves seekTimeUs, and on the next call to read()
extractor outputs all cues that should be displayed at this time and
later.
PiperOrigin-RevId: 395477127
