Use the new netcontext variants of DNS functions

This change should have zero impact on behavior.
This is an intermediate step toward DNS over TLS:
https://android-review.googlesource.com/#/c/380593/

Note that this depends on the Bionic change in
https://android-review.googlesource.com/#/c/399352/

Test: Existing netd tests continue to pass.
Bug: 34953048
Change-Id: Ie5d9a146e7945bcc56e3aceaf2ae4820c8901fed