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