SDK / exoplayer

1. Properly split out listening responsibilities so that
   DemoPlayer only listens to its own components.
2. Revert StreamingDrmSessionManager UnsupportedDrmScheme
   exceptions back to how they worked in V1, and inject
   a DrmSessionManager rather than a MediaDrmCallback into
   DemoPlayer.

This much better prepares DemoPlayer for promotion into
the core ExoPlayer library, since it removes assumptions
such as what SampleSource and DrmSessionManager impls
might be used with it.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122980952