HIDH: 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().

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Test: classic keyboard
Test: CtsVerifier HID
Change-Id: I5f79ad37862dbe9538745329e4a96f5d61d8b0e4