SDK / exoplayer

This is required because in V2 we'll be instantiating DRM
session managers before the ExoPlayer instance (and hence
we wont have the Looper). This logic will be further cleaned
up in later CLs that overhaul DrmSessionManager in more
depth.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122453765