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

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

Change-Id: Ibd3310421e7056675fb76eb940fdaacfb910599a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>