AppRTCDemo(Android): only stop the cameraThread's looper after stopping the camera.

R=glaznev@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20569004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6358 4adac7df-926f-26a2-2b94-8c16560cd09d