common_audio: Made input vector const in WebRtcSpl_LevinsonDurbin()

In addition, expanded the unit test to verify both unstable and stable filters.

BUG=3353, 1132
TESTED=locally on Mac and trybots
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/35599004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@8038 4adac7df-926f-26a2-2b94-8c16560cd09d
3 files changed