SDK / exoplayer

This was adding a lot of code, and the multiple use cases
for Downloader was pretty confusing (in particular the
ordering of method calls was unclear). It's also not
performant (e.g. it requires loading/parsing manifest(s)
and initialization segments from disk).

In practice I think apps will need to keep a record of
what's offlined in their app's database (or equivalent),
which they can update by registering as a listener on
DownloadManager. This will be done for the demo app in
a subsequent change.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194932876