The sample timestamp carried by the emsg box can have a significant delta when comparing to the earliest presentation timestamp of the segment. Using this timestamp to intialize the timestamp offset in TimestampAdjuster will cause the media sample to have a wrong adjusted timestamp. So we should defer adjusting the metadata sample timestamp until the TimestampAdjuster is initialized with a real media sample.
PiperOrigin-RevId: 538172841
(cherry picked from commit 08c189e768d77e9e2a9665b23fa384e776f9678b)
