Mark some PrivateDnsConfig members and methods @NonNull and @Nullable am: 2a6bebf2d6 am: 466bb2ea68

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

Change-Id: I24a33d2ec25758f40e7ef5e3ab35bff415b6a8af
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>