Mark some PrivateDnsConfig members and methods @NonNull and @Nullable am: 2a6bebf2d6 am: 7a2a82ea9f

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

Change-Id: I7269f2a3795b9885a85b1cf5f5fa80c9c0d94305
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>