added listener for events.

[garam-park]

I added code for observations.

Some times, I need handling start and end of animations.

Listener is used above.

    private ExpandableLayout expandableLayout;

    expandableLayout.setOnStartHidingListener(new ExpandableLayout.OnStartHidingListener() {
        @Override
        public void onHidingowing(ExpandableLayout layout) {

        }
    });

    expandableLayout.setOnStartShowingListener(new ExpandableLayout.OnStartShowingListener() {
        @Override
        public void onStartShowing(ExpandableLayout layout) {

        }
    });

    expandableLayout.setOnFinishHidingListener(new ExpandableLayout.OnFinishHidingListener() {
        @Override
        public void onFinishHiding(ExpandableLayout layout) {

        }
    });

    expandableLayout.setOnFinishShowingListener(new ExpandableLayout.OnFinishShowingListener() {
        @Override
        public void onFinishShowing(ExpandableLayout layout) {

        }
    });