Use IpUtils, NetworkStackConstants in DadProxyTest

The test had TODOs to address to use IpUtils and NetworkStackConstants
classes once they were moved to frameworks/libs/net. This is now done,
so using the new classes.

Bug: 158042941
Test: atest TetheringPrivilegedTests
Change-Id: I536b4506dbc3b42547ba9cb4e7c3c7a3e9459dd1