Use async private DNS resolution for privdns CTS

Enable networkmonitor_async_privdns_resolution for tests that change the
private DNS setting which are known to be flaky.

This avoids flakes when the private DNS configuration changes while
NetworkMonitor is evaluating it, by having NetworkMonitor stop and
restart resolving on configuration change.

Bug: 253698734
Test: atest
Change-Id: Ie3ac3af5351206e717baa07592efb6d176188228
2 files changed