Initial DBus transport support

This patch adds initial support for DBus transport on Linux. It exposes
initial implementation of Adapter that can be Enabled and Disabled. One
can also introspect the interface to find the exposed methods. Further
patches will extend it's functionality.

Test: ninja -C out/Default -j 40
Change-Id: I173cc752b8d8aaa8706ed36f75f5a043cc987b1a