Set Data SPN to SPN when not overridden

In order to allow applications to either always display
the SPN or the data SPN in various contexts, override
the Data SPN when it isn't otherwise provided.

Bug: 358669494
Test: atest CarrierDisplayNameResolverTest
Test: manually verified voice/messages/calling on Akita
      on P80481885 == AP3A.240905.013 + ag/29194106
Flag: EXEMPT bugfix
Change-Id: Ia8353a8c4035898ce524ae76157ef2637fb68aa9