cleanup - read_config() and ipv4_local_prefixlen am: 8452f3f652 am: d2ef8344cf

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/android-clat/+/11736257

Change-Id: Ib1bcf565debff06cc7610f549cd4175f9c6b064e