Bluetooth: OTC: Use ots_l2cap for l2cap transfer

Update the OTC to use the existing ots_l2cap code rather than its own
implementation.
- Add an ots_l2cap struct and register it
- Add ots_l2cap callbacks, reuse most of old l2cap_recv callback in
  the ots_l2cap rx_done callback
- Remove old l2cap code

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>