Address outstanding comments from aosp/713992

Test: as follows
    - built, flashed, booted
    - system/netd/tests/runtest.sh passes
    - connected to a DNS64 network and observed prefix discovery
      via "adb shell dumpsys netd"
Bug: 78545619
Change-Id: I7ae6bf258bb3f22795274026187ee8493875aace
6 files changed