Add stack/l2cap/l2c_api::l2c_get_transport_from_fixed_cid

Toward readable code

Bug: 163134718
Tag: #refactor
Test: act.py -tc BleCocTest
Test: ble paired 2 phones
Test: classic paired Bose SoundLink

Change-Id: I8032481fbd4ef6ed14b821e1e04ed5498aba10e9