ambient: Fix scale of album preview

This patch sizes the preview to cover the img element while keeping the aspect
ratio.

Bug: b/169434370
Test: manual
Change-Id: If54cd536d037e1cc9a6a75335fe1ea7e93d24a13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2442488Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812893}

chrome/browser/resources/settings/chromeos/ambient_mode_page/album_item.html

@@ -52,6 +52,7 @@
         border-radius: 4px;
         display: block;
         height: 160px;
+        object-fit: cover;
         position: absolute;
         transform: scale(1.0);
         transition: transform 240ms;