multi-track support upstream to the ChunkSource interface.
This change does not yet make use of the newly exposed APIs. This
will come in a subsequent CL.
Issue #514.
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
- Add constants class. Currently housing a single lonely variable,
which is used generally throughout the library, and so no longer
nicely fits into a specific class.
- Rename a few other constants to add clear units.
- Made minor tweak to ExoPlayer documentation.
