This makes the following changes to improve consistency among the PlaybackInfo
values:
1. Update buffered position and total buffered duration after loading period
is set as both values are affected by a loading period update.
2. Add copyWithPosition to allow updating the position without resetting the
loading period.
3. Forward the total buffered duration to playing position updates
as it may have changed with the new playing position.
Issue:#4899
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=215712328
d4b87cdc23