Add tests for strict mode private DNS validation.

Test successful and failed validation, and updating the config.
In order to do this, add a FakeDns class so we can change DNS
responses dynamically while the test is running.

Also a couple of minor fixes:
1. Make sure the DNS timeout is set. Before this CL, it was
   always 0. Not sure why. It does seem to be set to the default
   value (12500) when actually running on device. We didn't
   catch this because the only tests that use the timeout set it
   explicitly.
2. Make runNetworkTest a bit more realistic: always send
   NetworkCapabilities *before* calling notifyNetworkConnected.
   This is what ConnectivityService does.

Bug: 122652057
Test: atest FrameworksNetTests NetworkStackTests
Test: atest --generate-new-metrics 50 NetworkStackTests:com.android.server.connectivity.NetworkMonitorTest
Change-Id: Ifd6694262501874f3261c864a049cb35c6afb9c8
Merged-In: Ifd6694262501874f3261c864a049cb35c6afb9c8
(cherry picked from commit 89909bef)