Creating a C++ wrapper class for VAD

Also adding a mock. This work is part of an ongoing effort to
encapsulate encoders in AudioEncoder classes. The CNG encoder will also
be implemented as an AudioEncoder class, and will also contain a VAD
C++ wrapper.

BUG=3926
R=bjornv@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/27839004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7570 4adac7df-926f-26a2-2b94-8c16560cd09d