Fix clang-tidy misc-definitions-in-headers warnings

The flag checks if function and variable definitions in header files can
lead to potential ODR violations when headers are included from multiple
translation units.

Make a small refactor to resolve potential ODR violations, though
variables currently declared in resolv_fuzzer_utils.h does not have a
problem because the file is included by a single translation unit.

Bug: 259199981
Test: (in tm-mainline-prod branch) cd packages/modules/DnsResolver && mm
Change-Id: I780e5bd54f93be623a6cc97bb1afbd7c3de3b48a