[DRUEL04] Implement unsolicited NAT64 prefix event

Implement unsolicited NAT64 prefix event that will report to the
listener when change NAT64 prefix.

Bug: 173485754
Test: atest resolv_integration_test resolv_unit_test\
      resolv_stress_test resolv_stats_test_utils_test
Change-Id: If20353529969ffa956b56d19345519f286046e25