Make testVpnNetworkActive more deterministic.

This test is a bit brittle because it sets the underlying
networks while the VPN is undergoing validation by
NetworkMonitor. The test does attempt to disable validation,
but that's not actually possible - the only thing that's possible
is to tell NetworkMonitor to validate immediately without sending
any probes. So the underlying network change races with the
validation. I'm not sure why the test isn't flaky. It might be
because both the network change and the validation result in a
capabilities change, and the test expects "a capabilities change"
without expressing what change that should be.

Make this a bit more predictable by ensuring that the network
validates before the underlying networks are set.

This is useful because an upcoming CL will change the way
underlying network capabilities are propagated. With this test
CL, both the old and the new code pass.

Bug: 172870110
Test: test-only change
Change-Id: I319858228e8d097c0b60a107029f296385f91269
Merged-In: I319858228e8d097c0b60a107029f296385f91269
(cherry picked from commit 0b53357c)