HashMap.values does not guarantee a stable order, but our tests
assert that the order is stable. It's probably a nice property
that the order is the same as that in which downloads are added,
so this change makes the order stable, with that order.
A future change will likely improve the way this works, by
changing the internal thread "downloads" variable to have type
ArrayList<Download>, at which point the internal thread can just
pass a read-only copy of its list to the main thread whenever a
state changes.
PiperOrigin-RevId: 242460725
