Don't add padding for Smartspace (actual and preview).

As demonstrated in ag/27287153, the actual Smartspace doesn't have any
top padding, but the qsb_preview does.
  - Turns out, ShortcutAndWidgetContainer sets padding on its children.
  - The actual Smartspace has padding removed by overriding the
    `setPadding` method in SmartspaceViewContainer. (See ag/13562740.)

This CL updates ShortcutAndWidgetContainer.java to not add padding for
either the actual or the preview Smartspace. It makes use of
`R.id.search_container_workspace` to identify the applicable cases,
rather than checking class types or names. This way, there is no
Pixel-specific logic added.

Before: https://hsv.googleplex.com/6335187293044736?node=18
After: https://hsv.googleplex.com/5450651931770880?node=18
Actual Smartspace, for reference:
  - https://hsv.googleplex.com/6083634648842240?node=14

Note the `isChildQsb` also returns true for the QsbContainerView. Since
that class already has the same `setPadding` override:
  - https://cs.android.com/android/platform/superproject/main/+/main:packages/apps/Launcher3/src/com/android/launcher3/qsb/QsbContainerView.java;l=147-150;drc=e58aaf1802e43f73bbd417658c1c4c42e2fe28fd
This CL does't affect its padding.

Flag: EXEMPT bugfix
Bug: 338601960
Test: Go to Wallpaper & style preview for Home screen.
Change-Id: I5b6f7752cfee556b815c83e9df85f9e74f23ab4b