refactor(modal): change watch expression to check local binding

$$stackedMap data structure has a length() function to check number
of opened modals. 'modal-open' class on body is maintained
by $watch expression. It was monitoring openedWindows.length
which is always non-zero (since it's a function). $watch expression now
watches openedWindows.length() and hands this value to listener
callback local binding noOfModals.