Use target size to scale down a widget preview image

WidgetCells are being recycled in WidgetFullSheet. getWidth/getHeight
could be a recycled dimension. There is also no guarantee that measure
has taken placed before the scale down logic. Let's use the targeted
width for the scale down logic instead.

Test: Manual
Fix: 195417705
Change-Id: Idfb3cc485604d19658e210709bebde6f163003cf