[Sb refactor] Handle telephony display info with only an override

Manual testing (and many bugreports) show that sometimes we calculate a
network type of `Unknown` when it should actually be some form of
`OverrideNetworkType`. It is possible that this is due to differences in the way that the new pipeline calculates the icon group to use. If the
`TelephonyDisplayInfo` objects have a value of
`NETWORK_TYPE_UNKNOWN` for the `networkType` field but still have a
valid `overrideNetworkType`, then we would incorrectly calculate `Unknown` when we should use the override type.

Test: MobileConnectionRepositoryTest
Bug: 272784824
Change-Id: Ieaf30071cf2a0082dc005dfc8e8332c0a7456df6