Add new Interface properties "RoamTime", "RoamComplete", and
"SessionLength". "RoamTime" carries the roam time of the most recent
roam in milliseconds. "RoamComplete" carries True or False corresponding
to the success status of the most recent roam. "SessionLength" carries
the number of milliseconds corresponding to how long the connection to
the last AP was before a roam or disconnect happened.
Signed-off-by: Matthew Wang <matthewmwang@chromium.org>