Ensure validated networks connected before applying test URLs

Some tests use internal http server to mock the network
validation test URLs. The test URLs are expected to apply on
specific network only, like Wifi. However, the network may
sometimes unexpectedly reconnect due to network issues. If a
network is connecting while applying the test URLs, the network
will get the same mock validation result and carry the result
to the follow up tests. This will fail the tests after it.

Thus, verify the supported networks to be connected and validated
before changing the validation URLs to prevent the URLs using in
unexpected networks.

Bug: 248251876
Test: atest android.net.cts.ConnectivityManagerTest
Change-Id: I39428abe05ab4336ca49beec33b2f9973f14f239