SDK / exoplayer

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.

On HTC Butterfly these match, but Build.DEVICE is actually the right value to
use.

Issue: #377

dev -> dev-webm-vp9-opus

This will be needed for keeping WebVTT subtitle and separate
audio tracks is sync whilst loading.

Issue: #151

This OMX component is listed but can't be instantiated on this device.

According to the GitHub issue, some other devices are also affected, so these
will have to be added too.

Issue: #377