Add lifecycle tests to NetworkMonitorTest

- Ensure that NetworkMonitor quits when notifyNetworkDisconnected
  is called.
- Ensure that BroadcastReceivers are registered when a captive
  portal is detected and unregistered when the network validates
  or when NetworkMonitor quits.

Bug: 122164725
Test: atest FrameworksNetTests NetworkStackTests
Test: atest --generate-new-metrics 10 NetworkStackTests:com.android.server.connectivity.NetworkMonitorTest
Test: test fails if "Don't leak captive portal broadcast receivers." change reverted
Change-Id: I1202626face33cc1e1ec02016b95da487b3335c8