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: If4231804b77143b78f26a32c16db53fc4ac44cd8