MdnsResponses were only updated through
MdnsResponse.mergeRecordsFrom(MndsResponse), meaning that MdnsResponse
cannot be updated unless a MdnsResponse can be built from the update
packet.
Because a PTR record is required to build a MdnsResponse, this means
that it was not possible to add records to a MdnsResponse unless the
update packet contains a PTR record.
Fix this by having MdnsServiceTypeClient augment its current known
responses in addition to creating new responses from incoming
MdnsPackets.
Bug: 267570781
Test: atest NsdManagerTest#testPtrOnlyResponse with
mdns_discovery_manager_version set to 1.
Change-Id: Iba293b4772fcd79e94ee6412c9af8225a3349f12
42b1042f36