VideoCaptureAndroid: quit & join the camera thread on stopCapture.

Also fix latent bug where setPreviewRotation() wouldn't hold
the lock while its delegate setPreviewRotationOnCameraThread()
was running, allowing the camera to be freed between the
null-check and the use.

BUG=3389
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17619007

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6266 4adac7df-926f-26a2-2b94-8c16560cd09d