Make shadow drawable settable

Created by: joshfriend

If the panel is completely invisible initially, you may want to remove the shadow until the panel is visible. In the PanelSlideListener:

@Override
public void onPanelSlide(View view, float v) {
    switch (panel.getPanelState()) {
        case COLLAPSED:
            // Collapsed height is 0dp, so we don't want the shadow
            // showing unless the panel is visible
            panel.setShadowDrawable(null);
            break;
        default:
            panel.setShadowDrawable(
                getResources().getDrawable(R.drawable.below_shadow)
            );
            break;
    }
}