Measure WidgetsFullSheet before estimating the recommended table

size

Issue: The header view height was obtained before any measurement.

Test: On Pixel 5 4x5 gird with Photo widget shown in recommended
      table, make the device in landscape. Then open the full widgets
      picker. The widgets picker can be scrolled. Before this fix,
      scrolling is not possible.
Fix: 194172597
Change-Id: Ib30cf4b95d2ed23ff28f3d24e103f0e919b3a551