Implement roaming tracking in NetworkStats summary queries.

Whether a network is deemed roaming or not was already being tracked
as part of the NetworkIdentitySet, so the underlying data store
already tracks roaming and native data separately. However, this data
was being aggregated together in NetworkStatsCollection#getSummary,
since the NetworkIdentitySet is converted to an iface name for the
purposes of matching, and the iface name will be identical whether or
not the iface is considered roaming. Now it is separated.

Also fixes a long-standing bug in NetworkIdentitySet where an identity
read from a saved file would always be considered roaming == false,
even if it wasn't at the time it was written.

Bug: 25813438
Change-Id: I11ab5b51182ed8da7af8fde468df065f9fdc3dad