resolv_integration_test: Retrieve NAT64 prefix through DNS event listener

Currently, the unit test could retrieve NAT64 prefix through registering a
DNS event listener. It doesn't need to query the prefix through the binder
interface method getPrefix64() anymore.

Test: built, flashed, booted
      system/netd/tests/runtests.sh passes

Change-Id: I54386f7d8c03f7009c12fe3fc39da85985d8d3ad