SDK / exoplayer

- Support querying whether the current window is dynamic
  and seekable. The new methods are similar to getDuration,
  which is also a convenience method for the current window.
- Improve demo app to restore positions in VOD items within
  playlists where the last item is live. Also restore the
  position within the window for live items unless the player
  failed with BehindLiveWindowException.

Issue: #2320

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