fix(web): bounding rect offset was incorrect

When embedding the On Screen Keyboard into a page, the calculation for
the bounding rectangle only took into account the first offsetParent,
which meant it would get the offset incorrect when there were multiple
layers of offsetParents.

Once I changed to using getAbsoluteX / getAbsoluteY, this also corrects
for scroll with window.pageOffsetY, so we can eliminate that special
case for fixed positioning.

This was observable in the Web Developer test window, where interaction
with the bottom row of the keyboard in touch mode was not working
correctly, in 15.0.210-beta.