Add tests for always-on VPN lockdown mode.

This requires mocking lots of new things that weren't mocked
before but is otherwise fairly straightforward.

A few changes to MockVpn are needed as well:

1. Set the VPN's NetworkInfo to CONNECTED, so methods such as
   isBlockingUid will work. While I'm at it, set the interface on
   the LinkProperties as well to make things a bit more
   realistic.

2. Constructs the VpnConfig when registering the agent, not when
   the MockVpn is created. This is needed because starting and
   stopping lockdown VPN calls prepare, which nulls out mConfig.
   But constructing the VpnConfig when registering the agent is
   more realistic anyway. The production code does that in
   establish, but we can't do that in ConnectivityServiceTest
   because some of the test cases don't call establish and call
   registerAgent directly.

Bug: 173331190
Test: atest FrameworksNetTests
Change-Id: I827543751dbf5e626a24ec02cd6f50b423f5f761