SDK / exoplayer

DrmSessionManager is now an actual manager. For each session
request it may return a shared session (as things work now,
and as is suitable for Widevine VOD) or a separate instance
(e.g. for PlayReady where audio and video are protected with
different keys).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123203664