SDK / exoplayer

Currently our Renderer is always associated with and consume data from
some SampleStreams, which were constructed from the provided MediaSource.
There are use-cases, in which the users want to have simple Renderer
implementation that does not consume data from SampleStream at all, but
render using their custom logic at each rendering position - they mostly just
need ExoPlayer to keep track of the playback position and enable/disable the
renderer.
This CL adds support for such Renderer by adding a TRACK_TYPE_NONE.
Renderer of such type will be:
- Associated with null TrackSelection as the result of track-selection
operation.
- Associated with EmptySampleStream.

GitHub: #3212

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168545749