Fix runtime restart when using aggregated net observers.

When observing network traffic on a per-iface basis (as opposed to a
per-iface+uid basis), do not attempt to redistribute VPN traffic,
because it is unnecessary and will lead to a crash further down the
stack.

This is a one-line functional change, but some comments have been
added throughout to clarify behavior.

Bug: 29390363
Change-Id: Icca13f181d7698ca3ddc92aa9248f06f130d49a3