Add test coverage for strict mode private DNS.

Support faking out the DNS lookups used by NetworkMonitor to
resolve strict mode DNS, and add more test coverage.

These tests were partly adapted from tests we have in Q but
also contain new coverage. This is because in Q the interface
between ConnectivityService and NetworkMonitor changed
substantially, and it is impractical to backport
NetworkMonitorTest.

Bug: 122652057
Test: atest FrameworksNetTests
Change-Id: I6497b7efa539267576d38d3036eef0af0df4e9cb
Merged-In: Iaa78a7edcf23755c89d7b354edbc28d37d74d891