SDK / exoplayer

Step 6 of the refactor involves moving the logic that's
currently in the SourceBuilder classes in the demo app
into new SampleSource classes in the library. These classes
will construct video/audio/text pipelines on-demand (i.e.
when tracks are enabled) rather than constructing them all
up front as is currently the case in the SourceBuilder
classes. Hence we need a way to instantiate DataSource
instances (i.e. a DataSourceFactory ;)).
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