Use WidgetSizes to estimate widget size instead of custom calculation

Also fix 2 bugs in WidgetCell
1. preview layout: padding is added to the AppWidgetHostView rather
   than the preview container. We shouldn't add padding to the size
   of the preview container.
2. widget_preview_shortcut_padding is a padding added to shortcuts but
    not widgets.

Test: Open full widgets picker and observe widgets are rendered
      correctly in the widgets recommendation table. Also, observe
      the generated bitmap images are correctly rendered.
      Run AddWidgetTest.
Bug: 189975670
Change-Id: I00db3200e0b61dc5e82f3c4bfdf34e197ea20314