Add handler for runActionNamed

[hborisoff]

What	Where/Who
JIRA Issue	SDK-455
People Involved	@hborisoff

This change enables callback to be invoked when in-app message actions are run. For example - Accept action, Select button 1 action.

Added Dismiss action to be run for some of the in-app templates, where appropriate.

Example usage:

Leanplum.onAction("Center Popup", new ActionCallback() {
    @Override
    public boolean onResponse(ActionContext context) {
        context.setActionNamedHandler(new ActionCallback() {
            @Override
            public boolean onResponse(ActionContext context) {
                String actionName = context.actionName(); // "Accept action"
                String parentActionName = context.getParentContext().actionName(); // "Center Popup"
                return true;
            }
        });
        return true;
    }
});