Register VCN service in ConnectivityServiceInitializer

When VCN is in the Tethering module, the service registration should
be done via ConnectivityServiceInitializer. Specially it will only
register VCN when the device is B/B+. On older devices, VCN will
still be directly registered by the SystemServer.

Bug: 385203616
Test: atest CtsVcnTestCases && FrameworksVcnTests
Test: verified registration on Android B and Android V
Flag: RELEASE_MOVE_VCN_TO_MAINLINE
Change-Id: I3b80e9d131c9213942b724406261da19bda1a30d
2 files changed