Add one more Revalidation unit test

The test checks that the revalidation can retry probing until
it succeeds. To do that, Private DNS validation backoff time
is changable in PrivateDnsConfigurationTest.

Original change: https://android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/1708171

Bug: 79727473
Bug: 188153519
Test: Test: cd packages/modules/DnsResolver && atest
Change-Id: I11fc8d9ee963dc54d051667971b51bfecd852a93
Merged-In: I11fc8d9ee963dc54d051667971b51bfecd852a93