I think you need to at least call imageView.start() to play the video (assuming you have everything else set up properly).
You can also invoke installed Android video player to play the video with just a few lines of code.
Uri fileUri = Uri.fromFile(videoFile);
Intent intent = new Intent();
intent.setAction(Intent.ACTION_VIEW);
intent.setDataAndType(fileUri,
URLConnection.guessContentTypeFromName(fileUri.toString()));
startActivity(intent);
Intent chooser will show up to let user pick his preferred video player. I wrote a blog to discuss how to do it in detail, http://software.intel.com/en-us/blogs/2014/03/20/video-playing-with-android-media-player. Hope it helps.