Notify NetworkStatsService of roaming status changes.

When the roaming status of the cell network changes, it is handled as
follows:

-DcActiveState gets an event (either EVENT_DATA_CONNECTION_ROAM_ON or
 EVENT_DATA_CONNECTION_ROAM_OFF) and calls
 NetworkAgent#sendNetworkInfo with updated NetworkInfo.
-ConnectivityService then gets an EVENT_NETWORK_INFO_CHANGED and calls
 updateNetworkInfo with the new info.

With the previous code, the NetworkAgent's NetworkInfo object would be
updated internally, but because the state of the connection had not
changed from CONNECTED, the change would be considered a duplicate and
otherwise ignored.

With the new code, we call notifyIfacesChanged() when the state is the
same but the roaming status has changed. That should be all that is
necessary, as roaming is not exposed as a link property or network
capability today.

Bug: 26545456
Change-Id: I33d32d1fa84ce41dbe01542e5b498eda4b744b74