Before this change, the ad playback state stored the number of played ads in
each ad group. There was no way to represent that an ad had failed to load (and
it wouldn't be possible just to increment the played ad count to signal a load
error because there might be an unplayed ad before the ad that failed to load).
Represent the state of each ad (unavailable, available, skipped, played, error)
in each ad group. In a later change the player will use this information to
update its loaded MediaPeriods in response to future ads failing to load.
Also make the AdPlaybackState immutable and remove copying/duplication of its
fields in the ad timeline and period.
Issue: #3584
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183655308
b82178ecb4