A new playback nonce is created for each playback of the same item. Thus we
need to inject the nonce dynamically into the data source factory.
This CL adds the DataSource which does the actual insertion into the request
URLs and a MediaSource which listens to new media periods, to request the
nonce and to configure the data source factory for this media period to use
this nonce.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=197134217
2b9c31a14f