Conditionally accept loss of on-link IPv6 DNS servers

Permit this if and only if we're not "avoiding bad wifi" in general.

Bug: 31827713
Change-Id: I0874ec2d1d09e913ccdbe9f6e178090400808a04
(cherry picked from commit 5b437d6ff91116f25be946e07d5d90cd25c372a4)
2 files changed