Initialize callbacks in fuzzer

Fuzzer is crashing because no callbacks are registered for
DnsResolverService. Initiliazing calllbacks in DnsResolverService

Test: m resolv_service_fuzzer && adb sync && adb shell data/fuzz/x86_64/resolv_service_fuzzer/resolv_service_fuzzer
Bug: 269690691
Change-Id: I6b56bd7c79ccacfbc5a8ac7b4660084d891beff7