Measure WidgetsBottomPicker again when the number of spans changes

Bug: In onMeasure, we calculate the number of spans that can be fit
horizontally. If the number of spans has changed, we re-organize
widgets shown in the table. However, after that, we didn't measure
WidgetsBottomPicker and its children again. As a result, the newly
added widget previews' dimensions are never measured.

Test: Long press clock app and then click widgets. Rotates the screen
      and observe widgets correctly shown.
Fix: 191644950
Change-Id: Iae94fba5b137807483e061d785b18d59defb013e