Add doh_unit_test target

To prepare the doh test coverage.

Test: TH
Bug: 155855709

Original change: https://android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/1669634

Change-Id: Ib3ebaf7963aefcc45a5214600dd458aea3e571c6
(cherry picked from commit 4f6c1643)
Ignore-AOSP-First: To solve the confict between mainline-prod and other branches with newer toolchain.