SDK / exoplayer

- Changed startDownloads/stopDownloads back to their previous behavior
  of starting and stopping all downloads at the manager level.
- Made setManualStopReason methods for the new case of setting a manual
  stop reason or some or all downloads.
- Added plumbing to specify an initial manual stop reason when adding a
  new download, without also overwriting the manual stop reasons of all
  other preexisting downloads. Using the value is left as a TODO pending
  a bit of further cleanup that'll make it easier.

PiperOrigin-RevId: 242891688