Classic GAP_ConnOpen: Use L2cap to enforce security

Instead of using btm_sec_mx_access_request() to enforce security, let
L2cap enforce security for us through btm_sec_l2cap_access_req().

Note: Classic user L2cap channel (GAP_ConnOpen) API is not exposed.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: Ib848edb444d866eb70de9d7cf185ee8d45c71667