Fix Incorrect aria-modal Usage

[laurencepresland]

This PR sets the value of the aria-modal attribute based on the value of the blocking prop. According to the MDN Web Docs the aria-modal attribute is designed to limit screen readers to a particular area of an application. Therefore I think it makes sense that it should be set in line with the blocking prop.

Without this fix there is no way to set aria-modal to false therefore screen readers which respect the attribute (currently only iOS as far as I know) will always be stuck in the bottom sheet.

[alex-reid]

+1 on this. This is currently breaking accessibility on iOS devices if the bottom sheet is set to blocking={false}