resolv: export more structs from dns_responder

Move the struct DNSRecord and its relevant structs declaration to
the public header so that they can used to make a desired DNS answer
in other tests. No functionality change.

Bug: 134897288
Test: cd systen/netd && atest
Change-Id: I88bc145ee9b9173de56889acac11d15f46f1e445
2 files changed