[SB Refactor] Implement the wifi content description in the new

pipeline.

This is a re-implementation of the logic in:
 - WifiSignalController#notifyListenersForNonCarrierWifi
 - SignalController#getContentDescription

Bug: 238425913
Test: WifiViewModelTest
Test: manual: Logged content description and verified it matched old
system in variety of circumstances (full wifi, not full wifi, full wifi
that isn't connected yet, wifi off)

Change-Id: I4a730d117f497f4c608c73983122bc50f7065839