Save data registration roaming state

Some carriers require different APNs based upon
the roaming state returned in a previous registration
call.  The carrier config has the option to override
roaming state.  This change saves the proper roaming
state before the carrier config may override in order
to select the proper APN when setting up a data connection.

bug: 22990977
Change-Id: Id22a60efcbea9637018cc47e6d97f68eb053deb0