Improve HandoverRule check at roaming OOS

Even though the actual PDN is established at roaming area, if DUT moves
in OOS area, handover rules for roaming condition are not referred and
unwanted handover operation can occur.
To prevent this, Store last roaming state in datanetwork when DUT is
inService, and use it for handover rule check at OOS.

Bug: 284932261
Test: atest FrameworksTelephonyTests, DeviceTest: b/284932261#comment29
Change-Id: I32a81cf95e09ef7464b695828db264f1a393da7a