Replace isWeaklyValidatedHostname with guava

The guava implementation is more accurate than the basic NetworkUtils
implementation.
NetworkUtils is being moved to its own JAR and will not be available
from Settings.

Bug: 174436414
Test: m
      Wrote test for isWeaklyValidatedHostname, verified it passes with
      guava
Change-Id: I1d01f2beba36009e17f651652e2d7f57be5aa398