Extract adding/removing old/new values into methods.

java/src/main/java/lp/java/understanding/SlidingWindow.java

    public void slide(long slideBy) {
        windowStart += slideBy;
        removeOldValues();
        addNewValues();
    }
    private void removeOldValues() {
        for (int i = 0; i < currentValues.size(); i++) {
            long timestamp = currentTimestamps.get(i);
            if (!isInWindow(timestamp)) {
                i -= 1;
            }
        }
    }
    private void addNewValues() {
        for (int i = 0; i < newValues.size(); i++) {
            long timestamp = newTimestamps.get(i);
            if (isInWindow(timestamp)) {