Add java doc for registerVisibleActivityCallback method.

    /**

     * Registers a callback that will be notified when visible activities have been changed.

     *

     * @param executor The handler to receive the callback.

     * Note: The {@link VisibleActivityCallback#onVisible(VisibleActivityInfo)} will be called

     * immediately with current visible activities when the callback is registered for the first

     * time. If the callback is already registered, this method does nothing.

     *

     * @param executor The executor which will be used to invoke the callback.

     * @param callback The callback to receive the response.

     *

     * @throws IllegalStateException if calling this method before onCreate().