Null check added when updating the roaming status.

If the serviceState is null, calling getRoaming will cause a crash.
Instead of calling this method, set the string to not-roaming if the
value is null.

Test: atest -c SettingsUnitTest
Bug: 184334050
Change-Id: Ifa8a006838ea17f7d098c1b83bb6ab0349e7d569