L2cap LE Fixed channel shim layer

* Add the shim layer for LE Fixed channel to unblock testing
* Currently we use address type from advertising report or connection
  complete event. We need to use the address type from storage module in
  the future.
* So far GD ATT module is not implemented, and we always use legacy ATT
  cstack. We assign CID 4 to legacy ATT.

Tag: #gd-refactor
Test: nRF app
Bug: 158861440
Change-Id: I7c082201a481004f2340d22157beeb3808d1013a