VAD refactoring

* Added TODO comment
* Updated unit test
* Code style changes and added comments to set_mode()

BUG=
TEST=vad_unittests, audioproc_unittest, voe_auto_test

Review URL: https://webrtc-codereview.appspot.com/460006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@1943 4adac7df-926f-26a2-2b94-8c16560cd09d
3 files changed