Refine DNS private IP probes

This addresses comments on the parent change that introduced the
optional feature.
This change also considers IPv6 ULAs as private addresses, corrects some
style and comments.

Bug: 136734947
Test: atest NetworkStackTests:NetworkMonitorTest \
          NetworkStackTests:NetworkStackUtilsTest
Change-Id: I1169320f3a5c09631e754518c60acab337faf49b
Merged-In: If4231804b77143b78f26a32c16db53fc4ac44cd8
(cherry picked from commit 0f62e8c6, aosp/1282802)