Fix app widget preview layout drag-n-drop & rendering

1. Before this CL, preview layouts are not rendered in both the
   BottomWidgetSheet and SearchPopupSheet because the preview
   width & height are not set correctly.
2. Also applies the right padding to preview layouts.

Bug: 185374336
Test: Drag-n-drop preview layout from the full widgets picker,
      the bottom widgets picker and the search widgets picker.

Change-Id: I2b6303bfe4eb86a4935a5569f6107e9d46448902