Advertise only the implemented 32, 44.1 and 48 kHz stereo formats. Add
128-byte packets at 32 kHz and wrap the 44.1 kHz cadence at ten packets;
a uint8_t wrap at 256 otherwise emits extra samples every cycle.
Use the serialized UAC header length (including baInterfaceNr), maintain
the active alternate setting, accept alt 0 on non-streaming interfaces,
and retain the last valid sample rate when a SET_CUR request is rejected.
Change-Id: I41dab55fc0c7d2e6474492f97062e9838bc3aebf