Update VpnTest to handle teardown-on-network-lost

- Fix the wrong MOBIKE value set in
  testStartPlatformVpnHandlesNetworkLoss_mobikeEnabled.
- Verify the networkAgent is unregistered when the default
  network is lost

Bug: 231749077
Test: atest FrameworksNetTests:VpnTest
Change-Id: Idc6ebd42090eff1135dc5f96af4bc2fb268dd340
1 file changed