birch: update favicon style

This cl unified the favicon and app icon styles with small 16x16 icon
sizes and rounded background. For the tabs containing media, we will
use the new style until we can get the media artworks.

The cl also refactor the loading icon callback such that we can
easily add new icon styles in the future.

The modified icons are uploaded in the bug comment.

Bug: b:370805582
Change-Id: Ic7f105ae5df6b35e38ee2218a2e63d5ff77adad0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5943475
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1371555}