Don't assume MaybePaddedAndSlicedInput moved the padding into a kPad.

CudnnConvPaddingLegalization has logic to make sure that we call cuDNN with
the same low and high padding, and no interior padding (dilation). However it
then resets the padding in the window, even in the case when this padding was
not moved to a kPad. This CL now moves the update of the convolution window to
the same function that creates the kPad.

PiperOrigin-RevId: 224325347