stack::avct: Various cleanup

1. Expand avct initiator/acceptor enum syntax
2. Address cpplint errors
3. Update conventional logging
   a. Use 0x4 hex for l2cap cids
   b. Use hex for bitmasks
4. Remove dead code and unused definitions
5. Enumified key data types and added string representations
6. Statically scoped l2cap callbacks to translation unit
7. Highlighted issue with data narrowing on RC features

Bug: 373471940
Test: m .
Flag: EXEMPT, Refactor

Change-Id: Ie3719d2b318fe068c8185fddd8d190ca1c5eeef9