Let IpServer use RoutingCoordinator in R

On Android R, IpServer uses netd to set up tethering while on S+ it uses
RoutingCoordinator.

This CL lets Tethering own a RoutingCoordinatorService when the SDK
version <= R so IpServer can always use RoutingCoordinator instead of
netd.

Bug: 350699020
Test: CI

Change-Id: I0ed33fb050260c26be87079bed0e189716cd8782
6 files changed