Test: Use condition_variable to wait for onNat64PrefixEvent am: 4f4f57a8 am:...

Test: Use condition_variable to wait for onNat64PrefixEvent am: 4f4f57a8 am: df2490da am: ea02d3b0

Original change: https://android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/2825792



Change-Id: If0854c187f30e65fd8293250e0e713d25d79818d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>