Prevent VCN service from being registered twice

This patch is for migrating VCN registration from platform to
Tethering module. Becuase the system image has switched to use
mainline prebuilts, the mainline change cannot merged together
with the platform change. The plan is to update the mainline
prebuilt first and then merge the platform change. Thus there
will be a short window where ConnectivityServiceInitializerB
will be called twice to register VCN. This patch allows
ConnectivityServiceInitializerB to prevent the duplicate
registration.

This patch allows Tethering module to know whether VCN is moved
to mainline at runtime. If VCN is in mainline, Tethering
module (instead of SystemServer) will register the VCN service.

Bug: 385203616
Test: atest CtsVcnTestCases && FrameworksVcnTests
Test: verified registration on Android B and Android V
Flag: EXEMPT; no functional code change
Change-Id: If551167f4bafc39a592d8b325be3ceecf16f0b6e