SDK / exoplayer

This change pulls manifest refresh responsibility up to the
top level Dash/SS SampleSource implementations. In following
steps more of the manifest processing logic will be pulled
up (e.g. extracting track groups from the initial manifest),
which will allow ChunkSampleSource/ChunkSource instances to
be further simplified and created on demand.

I've avoided moving/renaming anything for now, so as to keep
it fairly easy to review.

Note that this change does the TODO related to releasing the
manifest fetchers.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=121001139