Add setupUdpSocket() and use it in send_dg()

When implementing mDNS .local resolution, it would have the exact
same code to setup a udp socket as send_dg(). Factor it into a
common helper function.

Bug: 140857615
Test: cd packages/modules/DnsResolver && atest
Change-Id: Id2a5945b85ea306ddbcc6ee84aa876ac0b7709fc