fix(modal): ensure document overflow styles are properly restored when multiple modals are closed/removed (#8390)

**Related Issue:** #6594 

## Summary

We store the initial overflow style to restore it when modal is closed,
which is only correct for the first modal that opens. Subsequent ones
will store the style that the modal applies, so the incorrect overflow
value is applied after all modals are removed in a different order.

This updates modal to store the initial overflow style only for the
first modal that is open. This will ensure the correct initial overflow
style is restored when all modals have been closed.

Author: jcfranco