samples: Bluetooth: Broadcast sink: Refactor audio RX path

Refactor everything related to RX in the sample.
The existing code will moved to new files with more specific
functionality, so that each file will become cleaner and use
fewer #ifdefs.

The USB part will be refactored a bit to follow the design used
by the LE audio shell, to make it easier to maintain both in
a similar way.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>