Merged success/failure callbacks into a single callback
in SignInCallback and AccessTokenCallback to simplify resource management and to avoid making copies.
PiperOrigin-RevId: 565744310
Details can be seen in the `Integrate reusable internal components into third_party` part of `server access design doc` (go/fast_pair_server_data_access)
PiperOrigin-RevId: 498208011