Remove hidden API usages backed by VcnTransportInfo

VCN will be moved to a mainline module and #getWifiInfo will no longer
be accessible. Also VcnTransportInfo will be a final class. This
patch updates SystemUI to use VcnUtils to get the WifiInfo and use
a Builder to construct a VcnTransportInfo instance for testing.

Bug: 369710077

Test: atest SystemUITests:MobileConnectionsRepositoryTest &&
      atest SystemUITests:ConnectivityRepositoryImplTest
Flag: EXEMPT pure refactoring
Change-Id: I255548e1d1c26e465657d8006272a689fc8a5de4