[sb] fallback from data_spn to spn

This is a tweak on recent changed behavior to move to SPN from DATA_SPN.
Instead of picking just one field from the intent, we implement a
fallback. This allows for carrier customization on the DATA_SPN field,
while still not failing in the case where DATA_SPN is null.

Test: MobileConnectionRepositoryTest
Flag: com.android.systemui.status_bar_switch_to_spn_from_data_spn
Bug: 358669494
Change-Id: I3cdf3566b2f3ee7359aa2f67d05535ec880ef6c0