CoreWindow::GetAsyncKeyState returns flags and not enums so checking
equality does not give the needed result.

As seen in qwindowskeymapper.cpp key events that only contain a modifier
key should not have the key itself as a modifier when calling
handleExtendedKeyEvent.

With the current approach the modifier states are the same as
on desktop Windows.

Task-number: QTBUG-58750
Change-Id: Ie0f3e1d3e8294e1a6b41c9223a7a5153306579f6
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>