Bluetooth: BAP: Shell: Add support for decoding LC3 data

Add support for decoding incoming LC3 data. At this point
we only instantiate a single LC3 decoder, so only one frequency
and duration is supported.

To accomodate for the increased RAM usage, the number of unicast
streams support have been decreased.

Further more, the LC3 handling in the shell has overall been
improved, also for encoding.

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