SDK / exoplayer

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

ChunkSampleSource had a null check solely for the VTT case,
where DashChunkSource wasn't setting a MediaFormat on VTT
chunks. It's trivial to do so, and is more consistent, so I've
done that and removed the null check. Also done some small
tidying.

parse mp4 tx3g

Merge webm/vp9 native extensions into dev.