Not abort when the function is not found in dns_helper.so

As the NDK API name is not final until reviewed by NDK council, it is
possible that after rolling out DnsResolver the methods would need to be
renamed.

Bug: 314893716
Test: manually change code to dlsym a non-existing function and there is
no crash.

Change-Id: I3da924c87a0e3995b8eb8fa8f4d1176a92fbc9e3