Disable warnings triggered in Clang r271374

http://b/28149048
http://b/29823425

Disable -Wvarargs that gets triggered in server/NetdConstants.cpp on
modules that are currently internal-only.  The second bug above tracks
that this warning gets re-enabled.

Change-Id: Iba51a413b32f238c915e14dd7696b97c2cb81d71