[SB Refactor] Fix handling of network transports in wifi repo.

Fixes:
1) Handle CELLULAR transport first, like the old pipeline.
2) Don't assume `NetworkCapabilities.transportInfo` is always of type
   `WifiInfo`.
3) Update `isWifiDefault` to match the old pipeline. (If the default
   network has the wifi transport, the wifi is default even if the
   `transportInfo` is not of type `WifiInfo`.)

Bug: 266628069
Test: atest WifiRepositoryImplTest
Test: manual: verify wifi icon still works
Change-Id: I47ab4eb01ee8a3f893acbf13a9a01bbd9fdee10d