SDK / exoplayer

- The HlsSampleSource now owns the extractor. TsChunk is more or less dumb.
  The previous model was weird, because you'd end up "reading" samples from
  TsChunk objects that were actually parsed from the previous chunk (due to
  the way the extractor was shared and maintained internal queues).
- Split out consuming and reading in the extractor.
- Make it so we consume 5s ahead. This is a window we allow for uneven
  interleaving, whilst preventing huge read-ahead (e.g. in the case of sparse
  ID3 samples).
- Avoid flushing the extractor for a discontinuity until it has been fully
  drained of previously parsed samples. This avoids skipping media shortly
  before discontinuities.
- Also made start-up faster by avoiding double-loading the first segment.

Issue: #3

dev -> dev-hls

Issue #81

Dev -> Dev-hls

Also tweak ManifestFetcher.

Pull dev -> dev-hls

Issue: #81

Issue: #81

Merge dev -> dev-hls

The key change here is that nextLoadPositionUs is set to -1
if we're not loading but don't have a next chunk ready to
load. This ensures that "missing chunks" in one stream don't
prevent chunks in another stream from loading. This occurs
in SmoothStreaming with TTML subtitles, where the chunks are
sparse.

Propagate elapsedRealtimeUs to the video renderer. This allows
the renderer to calculate and adjust for the elapsed time since
the start of the current rendering loop. Typically this is <2ms,
but there situations where it can go higher (normally when the
video renderer ends up processing more than 1 output buffer in
a single loop).

Also made variable naming more consistent throughout the package.

So that we actually catch it, rather than having the process crash!

Add file missing from "Improve subtitle handling" change.

Update dev-hls

- Move parsing onto a background thread. This is analogous
  to how frame decoding is pushed to MediaCodec, and should
  prevent possible jank when new subtitle samples are parsed.
  This is more important for out-of-band subtitles, which can
  take a second or two to parse fully.

- Add Useful DataSpec method.

Issue: #12

Previously we'd end up blocking forever in this case, which
is the worst thing we could do :). We could either throw an
exception or just print a warning. Printing a warning is more
in line with what other methods do (Handler prints a "sending
message to dead thread" warning).