Disable warnings triggered in Clang r271374

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

Disable -Wvarargs that gets triggered in server/NetdConstants.cpp.  The
second bug above tracks that this warning gets re-enabled.

Test: Tested build, boot and common usage for Arm, Arm64, x86, x86_64,
Mips images in AOSP and internal branch.

Change-Id: I5efea1897d7f7c2c1593475563b567503948b135