Mark some PrivateDnsConfig members and methods @NonNull and @Nullable am: 2a6bebf2d6 am: 316a3151fc am: 51c00f5426

Original change: https://android-review.googlesource.com/c/platform/packages/modules/NetworkStack/+/2843580

Change-Id: Ide5e53c5323794965bdec55bb398b1d5007c4aca
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>