Make legacy VPN work over stacked interfaces.

On stacked interfaces like 464xlat, Legacy VPN can't find the
default gateway because it uses getRoutes, which only returns
routes for the base link and not for the stacked links. It also
assumes that the interface that the default route points to is
the interface for the base link (e.g., rmnet0) instead of the
interface the route actually points to (e.g., clat4).

Fix this by calling getAllRoutes to find the default IPv4 route,
and get the interface name from the route we find instead of
assuming it's the base interface.

Bug: 9597516
Change-Id: Ia6ce0b6258a421cd22f60dedca7e94176b32176b