SDK / exoplayer

Mechanical step to create a new HlsSource in the library.
Note that naming is now confusing. This will be fixed in
the next CL, when:

HlsSource -> HlsSampleSource
HlsSampleSource+HlsOutput -> HlsOutput
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122054800