WidgetCell & LiveWidgetPreview cleanup

The fixes are:
1. For widgets shown in TableRow, aligns them center vertically.
   This is useful when widgets of different height are shown in
   the same row.
2. Remove background color from WidgetCell
3. Re-enable swipe in FullWidgetsSheet
4. Use the WidgetCell image as the single & long press area because
   1. Previews are now scaled to their default size. The touch area
      should already be larger for widgets that are bigger than 2.4
      cells
   2. WidgetCells also render description. If the user long press the
      description area, the drag view will not align in the middle of
      the user touch / drag area.

Test: Drag-n-drop widgets from FullWidgetsSheet, BottomWidgetsSheet
      and pin widget flow.
      In FullWidgetsSheet, swipe left and right to switch work /
      personal tab.

Video: https://drive.google.com/file/d/1Ur7PwF1a7iwGGRTZczfc0BSVu-Ta6-Vm/view?usp=sharing

Bug: 179797520
Change-Id: I6fe4bc88d1e1b35b1819d8d7f046105f5ed27043