Introduce bluetooth::legacy::hci::Interface

Migrate away from globals which allow any module
anywhere to bypass design intent which may
potentially introduce unintended code paths and
possible instability in module relationships.

Also towards testable code

Bug: 163134718
Tag: #refactor
Test: gd/cert/run --host

Change-Id: I78a06be73906973e0bbdaba78cc7fbf27d9b27cb