Show widget previews in their expected cell sizes

1. Show widget previews in their expected cell sizes.
2. Update the bottom picker to use a table layout rather than
a horizontal scrollbar.
3. Move the title and cell size description to the bottom of
the preview.

Test: Run AddConfigWidgetTest
      Manually open full and bottom widgets picker and observe
      widget previews.
      Video: https://drive.google.com/file/d/1K8A8hK5B4FgPTsX3cb-Yt4a_VjI0pk-w/view?usp=sharing
Bug: 179797520
Change-Id: I938612cb2e2477ce4a3a2d6c8891a0a36fc0978c