DNS64: return both synthesized IPv6 address and queried IPv4 address

Return both synthesized AAAA records and queried A records when
AF_UNSPEC is specified.

Bug: 159305929
Test: unit test
$ atest resolv_integration_test

Test: manual test:
query a domain name which has A record only

- Prepartion:
  i. reboot
  ii. connect to IPv6 only network (GoogleGuest)
  iii. block ipv4.google.com AAAA dns query
    $ adb shell "ip6tables -A OUTPUT -p udp --dport 53 -m string --algo bm --hex-string \"|04 69 70 76 34 06 67 6f 6f 67 6c 65 03 63 6f 6d 00 00 1c 00 01|\" -j DROP"
  iv. build and push dnschk to device
    $ make -j4 dnschk
    $ adb push out/target/product/raven/system/bin/dnschk /data
    $ adb shell chmod 755 /data/dnschk

- Test:
  case A: without this commit --> IPv6 only (as expected)
  $ adb shell /data/dnschk ipv4.google.com
  # ipv4.google.com (via nethandle 0):
  64:ff9b::8efb:2aee

  case B: with this commit --> IPv6 + IPv4 (as expected)
  $ adb shell /data/dnschk ipv4.google.com
  # ipv4.google.com (via nethandle 0):
  64:ff9b::8efb:2aee
  142.251.42.238

Change-Id: I4e2f88d0fe961313c40af7cd1e54766029cf5f91