Update FrameworksNetTestsLib dependencies on VPN

In change ID: Ia2279bd1023296e3c949fa05bd74e921bf17da79,
ConnectivityServiceTest dependencies on VpnManagerService were removed,
adding a more limited dependency on LockdownVpnTracker instead. This
updates the "keep" rules for services.core-vpn to reflect that.

services.core-vpn is a subset of services.core that only includes the
classes in the "keep" rules, and their dependencies. This is because
ConnectivityServiceTests still depends on internal VPN classes, but
other usage of hidden API should not be added to the tests.

Using VpnManagerService causes problems for calculating coverage, as it
has indirect dependencies on many classes that end up included in the
test and show as uncovered in the dashboards including
NetworkPolicyManagerService.

The dependency on NPMS was found by looking at coverage reports and
unzipping services.core-vpn.jar. The fact that it is included due to
indirect dependencies from VpnManagerService was discovered by running a
dependency analysis tool (https://depfind.sourceforge.io/) on
services.core-vpn.jar in the out/ directory, with:

$ bin/DependencyExtractor services.core-vpn.jar > /tmp/graph.xml
$ bin/c2c /tmp/graph.xml -show-inbounds > /tmp/inbounds.txt

And running a script on the /tmp/inbounds.txt graph to find the path
from VpnManagerService to NPMS.

Bug: 259632671
Test: m
Change-Id: Idc6c9e6e998dbf968fd97582f904cb7aed56b5b5