Fix for tapping alert view buttons on iPads

On iPads, alert views are inside a `_UIAlertNormalizingOverlayWindow` whose coordinate space is not the same as that of elements' `accessibilityFrame`s.  This commit converts the accessibility frame into the main window's coordinate space instead, allowing alert view buttons to be tapped on iPads.

Also fixed is a bug whereby a tap step would seem to succeed if `tappablePointInRect:` returns `(NAN, NAN)`, which is what currently happens on iPads, even though the element is never tapped.