- 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
