service: Introduce bluetooth::Adapter::Observer

This CL introduces a new Observer interface for the Adapter class.
This meant to serve as the top-level API for receiving Adapter state
notifications, property updates, etc, in a format that doesn't use
the native HAL types.

Bug: 23328384
Change-Id: Ibf29063a169ad5e986d433d0901b3b973af0aa56