personalization: refactor PUB main page elements

Clean up wallpaper_preview and ambient_preview_large elements so that
the HTML files are easier to follow.

BUG=None
TEST=browser_tests --gtest_filter="*PersonalizationApp*"

Change-Id: I3533d87909b70bbddc6d883f89c335ef1227db54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4548360


Commit-Queue: Jerry Liu <pzliu@google.com>
Reviewed-by: Jeffrey Young <cowmoo@google.com>
Cr-Commit-Position: refs/heads/main@{#1146741}