When getMedatadata from an ExtractorMediaPeriod, due to the MediaPeriod refreshing its Timeline twice, the MetadataRetriever may wrongly reset periodIndex of the current trackGroup to C.TIME_UNSET, which makes it unable to handle subsequent metadata queries.
This CL changes the MetadataRetriever to keep track of periodUid of the current trackGroup instead of period index. PeriodUid is not changed on timeline refreshing, so is not prone the existing problem.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=201969350
