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.
(cherry picked from https://android-review.googlesource.com/q/commit:d6b2e5bf3f35345b040d5e738786fe8154393fef)
Merged-In: I3da924c87a0e3995b8eb8fa8f4d1176a92fbc9e3
Change-Id: I3da924c87a0e3995b8eb8fa8f4d1176a92fbc9e3