- Listener based reporting of progress allows the content length
to be persisted into the download index (and notified via a
download state change) as soon as it's available.
- Moved contentLength back into Download proper. It should only
ever change once, so I'm not sure it belongs in the mutable part
of Download.
- Made a DownloadProgress class, for naming sanity.
PiperOrigin-RevId: 244242487
