Controls are still hidden while playing ads, but if the app pauses the player,
controls will be shown. During ads, the player is not seekable.
When the player enters the background then returns to the foreground, the
content period may not be prepared, so also cache the content window duration.
This means that if the app reenters the foreground while an ad is paused the
time bar can be populated.
Issue: #3303
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171123428
