Use dlopen() and dlsym() for resolver functions

This is a workaround to allow us to use libnetd_resolv.so from
the resolver APEX before b/120661824 is fixed.

Test: builds, boots
Test: system/netd/tests/runtests.sh
Test: atest FrameworksNetTests android.net.cts.ConnectivityManagerTest
Change-Id: I752ae248bb59545b0a4fbc1a7b8c9bcd697a31d4