Make it obvious that connection_manager is separate from GATT

Even though it now shares client identifiers with GATT, the plan is to
have it as independent piece of stack.

connection_manager is also used from L2CAP, and it looks improper when
l2cap relies on some GATT related things.

Test: mma -j32
Bug: 372202918
Flag: EXEMPT, just renaming/moving code
Change-Id: I7ae54072f140c360edac64ab4e1c9bcef72b1695