Export callback object as a D-Bus object

This adds the first example of how to export a callback object as a
D-Bus object implementing an interface. Exporting the callback may also
utilize the existing D-Bus projection utility that is used by the
server.

Bug: 188718349
Tag: #floss
Test: manual - Build floss on Linux

Change-Id: Ibbb0a115e7671a32eb04db370148005b3693bad8