[Sb refactor] Use MobileMappings key for Unknown net types

ResolvedNetworkType.UnknownNetworkType was using "unknown" as the key,
but MobileMappings actually can potentially have some embedded
information about which network type icon to display in the case of
unknown. This would also always cause a lookup failure, causing the G
network indicator to show in cases where it should not be.

Test: MobileConnectionRepositoryTest
Bug: 269040632
Change-Id: I61f450faa756487983f300ff9dfefdcb74424e43