Test: Fix a flakiness in PrivateDnsConfigurationTest

Guard all the code in onValidationStateUpdate to ensure
`mObserver.serverStateMap` is updated before the tests check the
map.

Bug: 283705283
Test: ran resolv_unit_test 1000 times on HWAsan device
Change-Id: Ia1fbca01c8d2fbde3b32272d2ed3522baf3b452a