Add a bottom padding for the last feed card

This is used to work around the problem that the last divider
overlapps with the bottom edge of feed containment.

Phone:
Before fix: https://screenshot.googleplex.com/5iqFu7dtiJr5KBS
After fix: https://screenshot.googleplex.com/Az3RNYcLvE5mM8o

Tablet:
Before fix: https://screenshot.googleplex.com/4Waim37TxPQNhaw
After fix: https://screenshot.googleplex.com/6UBwpFXZ9nF5x3T

Bug: 345688430
Change-Id: I4c17e360118cc3d878f1c6abaf5d033197ea1120
Low-Coverage-Reason: no need to test resource change
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5609415


Reviewed-by: Xi Han <hanxi@chromium.org>
Commit-Queue: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312898}