MdnsResponse would not replace records if the TTL changed, but this is
necessary in particular in case of exit announcements (RFC6762 10.1)
where the TTL is updated to zero.
Update MdnsResponse to update records if the TTL changed. The
receiptTimeMillis is still not considered to update records, but doing
so would generate service change notifications every time a reply is
received, so this is not covered here.
Bug: 267570781
Test: atest MdnsResponseTests
Change-Id: Ied55829cba6b043d13603a8230bda457c07de42f
36981e2352