Factor out response decoding into MdnsPacket

Factor out generic packet decoding into MdnsPacket, out from
MdnsResponseDecoder.

This allows reusing the same decoding code for replies, and other kinds
of MdnsPackets.

Bug: 241738458
Test: atest MdnsResponseDecoderTests MdnsPacketTest
Change-Id: I4380f80240ed5a367accfc1b0c595967ee475578
4 files changed