Remove assertions in pauseAd()/playAd(), which can fail if events are delivered
after detaching the player, and log warnings instead.
Use whether IMA has sent CONTENT_PAUSE_REQUESTED/CONTENT_RESUME_REQUESTED to
determine whether we pause/resume the AdsManager, matching the IMA
documentation.
Also clean up use of player.isPlayingAd vs playingAd.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162364751
