Mark some PrivateDnsConfig members and methods @NonNull and @Nullable am: 2a6bebf2d6 am: 466bb2ea68 am: 078e0a42df

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

Change-Id: I9e58dc9772a4e1f0f148a6f8dde662a8c91e50fa
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>