Restart preview after each takePicture is done.

Preview is called when CAMERA_MSG_COMPRESSED_IMAGE is recieved by
dataCallback(). It needs to be started as a new thread so that the
callback can return, and the camera can know that takePicture() is
done.

Change-Id: I4d0febbc993aac43b37f9f4a824e9c7b8785f19e