SDK / exoplayer

* Split findNextCueHeader and validateWebVttHeader into static methods.
  This is a step toward WebVTT in HLS, where we'll need to re-use these
  to peek at the top of the WebVTT file (they'll be moved into a util
  class).
* Made parser robust against bad cue headers + added a test.
* Removed spurious looking assertion in WebvttSubtitle.