方案一：感谢@xjxuvup 提供

鉴于多数人都用过ThumbnailUtils.createVideoThumbnail()方法，该方法在2.x系统下可用，API LEVEL > 14时却只能返回null，以下为解决该问题方案：

重写createVideoThumbnail()方法，如下

@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)

private Bitmap createVideoThumbnail(String url, int width, int height) {
Bitmap bitmap = null;
MediaMetadataRetriever retriever = new MediaMetadataRetriever();
int kind = MediaStore.Video.Thumbnails.MINI_KIND;
try {
if (Build.VERSION.SDK_INT >= 14) {
retriever.setDataSource(url, new HashMap());
} else {
retriever.setDataSource(url);
}
bitmap = retriever.getFrameAtTime();
} catch (IllegalArgumentException ex) {
// Assume this is a corrupt video file
} catch (RuntimeException ex) {
// Assume this is a corrupt video file.
} finally {
try {
retriever.release();
} catch (RuntimeException ex) {
// Ignore failures while cleaning up.
}
}
if (kind == Images.Thumbnails.MICRO_KIND && bitmap != null) {
bitmap = ThumbnailUtils.extractThumbnail(bitmap, width, height,
ThumbnailUtils.OPTIONS_RECYCLE_INPUT);
}
return bitmap;
}

之后再自己的后台线程中调用该方法得到网络视频的缩略图bitmap然后在主线程中调用imageView.setImageBitmap(bitmap)即可；