public Builder cacheDirectory(File file) {
            this.cacheRoot = checkNotNull(file);
            return this;
        }
Overrides default cache folder to be used for caching files.
<p>
By default AndroidVideoCache uses
'/Android/data/[app_package_name]/cache/video-cache/' if card is mounted and app has appropriate permission
or 'video-cache' subdirectory in default application's cache directory otherwise.
</p>
<b>Note</b> directory must be used <b>only</b> for AndroidVideoCache files.

@param file a cache directory, can't be null.
@return a builder.
VideoCache/src/main/java/com/yc/videocache/HttpProxyCacheServer.java