How to clear webview cache when server side content updated?

Question

I have a HTML cache enabled WebView app that sends Javascript fetch request to server and gets images list from server, images on server updating every random hour like: today 11:00, 16:00, tomorrow 14:00, 19:00. Images count is always 20 and their names are always like: 0.png, 1.png, 2.png,... 20.png.

Problem is once when app caches images with names like that, after replacing/updating them with other same name images Web app is not showing them, shows just old first time cached images. I know that i can pass get parameter in javascript code while sending get request like: "site.com?time="+Date.getHours(); But i need cache update only on image update on server

Tried code below to clear cache every day but that is not enought:

//helper method for clearCache() , recursive
//returns number of deleted files
static int clearCacheFolder(final File dir, final int numDays) {

    int deletedFiles = 0;
    if (dir!= null && dir.isDirectory()) {
        try {
            for (File child:dir.listFiles()) {

                //first delete subdirectories recursively
                if (child.isDirectory()) {
                    deletedFiles += clearCacheFolder(child, numDays);
                }

                //then delete the files and subdirectories in this dir
                //only empty directories can be deleted, so subdirs have been done first
                if (child.lastModified() < new Date().getTime() - numDays * DateUtils.DAY_IN_MILLIS) {
                    if (child.delete()) {
                        deletedFiles++;
                    }
                }
            }
        }
        catch(Exception e) {
            Log.e(TAG, String.format("Failed to clean the cache, error %s", e.getMessage()));
        }
    }
    return deletedFiles;
}

/*
 * Delete the files older than numDays days from the application cache
 * 0 means all files.
 */
public static void clearCache(final Context context, final int numDays) {
    Log.i(TAG, String.format("Starting cache prune, deleting files older than %d days", numDays));
    int numDeletedFiles = clearCacheFolder(context.getCacheDir(), numDays);
    Log.i(TAG, String.format("Cache pruning completed, %d files deleted", numDeletedFiles));
}