SDK / exoplayer

After the fix in https://github.com/mockito/mockito/issues/2331, the calls to
equals on the fake transfer listener (due to its use in a list of listeners)
are treated as interactions with it, meaning that the current verification of
'no more interactions' will fail.

This change makes the transfer listener used for testing count bytes then
delegate to another (mock) transfer listener that's passed in to avoid the
problem.

PiperOrigin-RevId: 391949619