Wrap the on_finished_playback CallbackManager storage, registration
method and call site behind USE_RTTTL_FINISHED_PLAYBACK_CALLBACK so
configs without an on_finished_playback automation pay zero RAM and
zero flash for it.
Keeps CallbackManager rather than switching to StaticCallbackManager
because rtttl is MULTI_CONF — a per-class compile-time N would force
every instance to reserve max(callbacks_per_instance) slots.