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

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

Change-Id: I072d8dbf7629138d98306b023fc58a94c6ff08c8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>