SDK / exoplayer

1. Remove requirement for TrackRenderer implementations to report
   current position, unless they are time sources.
2. Expose whether renderers have media to play. The immediate benefit
   of this is to solve the referenced GitHub issue, and also to only
   display the appropriate Audio/Video/Text buttons in the demo app
   for the media being played. This is also a natural step toward
   multi-track support.

Github issue: #541

- Fix bug where duration of initially disabled tracks wasn't correctly
  incorporated into the overall duration reported by the player.
- Don't transition to STATE_ENDED unless the position has reached the
  duration, if the duration is known. This allows for "playback" to
  continue when all renderers are disabled, rather than jumping straight
  to STATE_ENDED.

Github issue: #554

This fixes SmoothStreaming on AndroidTV, and also removes a
warning that gets logged when using Widevine/FMP4.

- contentType can be defined on an AdaptationSet.
- language can be defined either in AdaptationSet or in a contained
  ContentComponent.
- The id from the AdaptationSet should come from the AdaptationSet.

to if the playlist load fails.

NB - I'm aware the casting is a bit messy, but I don't want a common
interface because I'm hopeful that TsChunk will go away at some point.

Issue: #537

Fetch next Dash manifest using URL in Location directive if exists

This makes it so that it's no longer necessary to specify the number
of downstream renderers to HlsSampleSource, FrameworkSampleSource and
ExtractorSampleSource, by forcing the downstream renderers to register
with the SampleSource instances in their constructors. This eliminates
a common source of subtle client bugs where the passed value is incorrect.

For enabling during device test runs.

Issue: #530

Subrip (SRT) support