dbus: expose bus connection unique name

Add an API to Bus class to expose the unique name of the bus connection.
The function will return an empty string if bus connection is not
established.

BUG=chromium:507718
TEST=unittest

Review URL: https://codereview.chromium.org/1222073003

Cr-Commit-Position: refs/heads/master@{#337732}
3 files changed