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:NetworkControllerWifiTest
Flag: Exempt pure refactoring
Change-Id: I66b71138fc0f2a746264aea551dfc7fef2f21526