Currently only the buffered position in the current media period can be queried.
To achieve this, we save the buffered positions of all MediaPeriods to the
PlaybackInfo together with a list of MediaPeriodIds. ExoPlayerImpl can then
determine the correct buffered position for multi-period windows and windows
with midroll ads.
In addition, this change adds two new convenience methods to the Player interface
to query the total buffered duration across all windows and to get the buffered
duration of the content while playing an ad.
Issue:#4023
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=200041791
c1181000f9