[DRUEL01] Add DnsResolverUnsolicitedEventListener

Add DnsResolverUnsolicitedEventListener that can register
unsolicited event listener to get some dns resolver info.

Bug: 173485754
Test: atest resolv_integration_test resolv_unit_test\
      resolv_stress_test resolv_stats_test_utils_test
Change-Id: I5c229b3f23d5779f289d72f622b3c894780bc417