Add the DNS query hook to net context

This change
 - adds a query hook to android_net_context
 - exposes relevant definitions to netd
 - corrects a bug in query hooks' interaction with the cache

This change does not introduce any code to read the query hook
from the net context or make use of it.

Bug: 34953048
Test: Netd test suite passes
Change-Id: Ie091980e22ce9da07a3c4d387b371e544379d762