Add VTI Netlink Interface in NetD

Includes the logic to add / update / remove Virtual Tunnel Inteface
in RouteController.

Bug: 63589711
Test: Ran runtest -x "system/netd/tests/netd_integration_test.cpp"
Change-Id: I49a86f2eceecaf34fbb1e5dc05a888447437bef0
diff --git a/server/XfrmControllerTest.cpp b/server/XfrmControllerTest.cpp
index b883dce..5715a6b 100644
--- a/server/XfrmControllerTest.cpp
+++ b/server/XfrmControllerTest.cpp
@@ -671,5 +671,7 @@
 
 }
 
+// TODO: Add tests for VTIs, ensuring that we are sending the correct data over netlink.
+
 } // namespace net
 } // namespace android