[part-1] Perform doMeasure only once in WidgetsFullSheet

We already know the details used to calculate the insets while in
onMeasure, so, instead of measuring everything twice, we initialize
the maxSpansPerRow in onMeasure and then call doMeasure.

Bug: 346341156
Flag: EXEMPT bugfix
Test: Manual and screenshot
Change-Id: If080f1f91c69d2d54139d876c36eac8743a224ee