Remove hidden VcnTransportInfo methods from VpnTest

VCN will be moved to a mainline module and the constructor will
not be accessible to VpnTest anymore. This commit updates VpnTest
to use the Builder that will be exposed as SystemApi

Bug: 369710077
Test: presubmit
Flag: EXEMPT pure refactoring
Change-Id: I6217c8312140b51ebe746c5fe0b12a5ced428f6c