Add varargs methods to build DhcpServingParams

In common use-cases, only one gateway or DNS server may be set, so the
added methods make it easier for clients to build the parameters.

Bug: b/109584964
Test: runtest --no-hidden-api-checks frameworks-net
Change-Id: Ib54bc60a54ad4ef0605baea62d87b9f14caf8aa7
2 files changed