SDK / exoplayer

- Make a best effort to avoid posting MSG_DOWNLOAD_THREAD_STOPPED
  if the internal thread has already quit. Doing so is harmless,
  but causes an error in Logcat. We used to generate this warning
  quite a bit in ExoPlayerImplInternal as well, and we got quite
  a few issues filed about it, so best to avoid it if possible.
- Null out the back reference DownloadThread holds to its manager
  on release. This avoids a potential issue where a download thread
  can prevent GC of its manager if it doesn't cancel quickly.
  There's similar logic (with a similar comment) in Loader.LoadTask.

PiperOrigin-RevId: 243365143