When reading the last period, the readingPeriodHolder was set to null in
updatePeriods if it was the last period. (This would occur almost immediately
when playing a single-period source.) seekToPeriodPosition suppresses reusing a
loaded/prepared period if the reading period and playing period did not match,
which meant that the whole timeline was recreated when seeking in the last
period.
Leave readingPeriodHolder non-null. This means that at all times either
playingPeriodHolder == readingPeriodHolder (and they could be null or
non-null), or playingPeriodHolder and readingPeriodHolder differ and are both
non-null.
Also fix an issue where streams were never forced to be recreated during track
reselection when reading ahead.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=134774238
