Simplify MockVpn.

This CL removes four methods in MockVpn by slightly changing the
test code to leverage the actual methods implemented by the
(production) Vpn superclass.

This works because setting mInterface results in
isRunningLocked() returning true, which makes a number of methods
behave as if the VPN is connected (which is what the test
expects).

The more realistic behaviour exposes a minor bug in the treatment
of underlying networks. Add a TODO to fix it.

Bug: 173331190
Test: test-only change
Change-Id: I49421183538ba61ca790af71e309ece36b653bf9