Fix use-after-free in DNS64 discovery thread

DNS64 discovery thread is detached from binder requesting thread. But
the discovery thread references resources not belongs to itself, which
can be destroyed in dnsresolver destruction.

Holds a strong pointer of Dns64Configuration in DNS64 discovery thread
so that the instance of Dns64Configuration will keep until the DNS64
thread is force terminated.

Ignore-AOSP-First: Fix security vulnerability
Bug: 278303745
Test: atest
Change-Id: Id74ea4e6f54a00805d3cc8a9d7e15e58a473b7d3