Register VCN in ConnectivityServiceInitializer when it is in mainline

When VCN is in the Tethering module, the service registration should
be done via ConnectivityServiceInitializer

Bug: 385203616
Test: atest CtsVcnTestCases && FrameworksVcnTests
Test: verified registration on Android B and Android V
Flag: RELEASE_MOVE_VCN_TO_MAINLINE
Change-Id: Ie24f9002740cdb43ed308f8a92c8f131fd9974d7