SDK / exoplayer

- Remove assertReleased and replace it with a proper condition variable
  that's opened when Downloader.download or Download.remove finish. As
  far as I can tell assertReleased was basically implementing "sleep for
  10 seconds after the Downloader starts". Note fixing this properly
  also makes the tests run much faster!
- Use ConditionVariable instead of CountDownLatch(1).
- Use AtomicInteger instead of volatile int because it's clearer and
  allows removal of explanatory comments.

PiperOrigin-RevId: 308819204