Handle aria-current=false as False instead of True (#7901)
* Handle aria-current=false as False instead of True
* Fix docstring for NVDAObject._get_isCurrent
* Address review comments
* isCurrent defaults to None instead of False, so now it returns either
None or one of the allowed strings ("true", "page", etc)
* Use the search method on the regular expression to match the
aria-current attribute in Edge, so it is also found if it's not the
first attribute in the string. Also, document that false is a valid
value.
* Improve comment about aria-current regex in Edge
