Reduce top padding of Taskbar's All Apps tray.

The top padding looks too high because the code is assuming we have a
search bar at the top of the bottom sheet. Until we implement search, we
should temporarily reduce padding for the taskbar implementation.

Before:
  - No tabs: https://screenshot.googleplex.com/AAPdR5nbwepWQ82.png
  - Tabs: https://screenshot.googleplex.com/3DnQp6PKhqXyRag.png

After:
  - No tabs: https://screenshot.googleplex.com/A8enwnSs8r78ccj.png
  - Tabs: https://screenshot.googleplex.com/A3fAy2bXgmSPrG9.png

Test: Manual
Bug: 216683257
Change-Id: Ie0837acc3aed162b9428a209962be2a102c515c8