[Start] Add divider line to Feeds placeholder.

Before:
https://drive.google.com/file/d/1VSFkQ7afBF-qhn-k9AwpiR9heKSqkmBC/view?usp=sharing
After:
https://drive.google.com/file/d/1CvzqqFg_K2fl33KxZiPumI-vz8GIUVSy/view?usp=sharing

(cherry picked from commit 13b12a5)

Bug: 1328940
Change-Id: Ie92f408c15afff645715ea6b2eefa047267e5372
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3665500
Reviewed-by: Cathy Li <chili@chromium.org>
Reviewed-by: Hao Dong <spdonghao@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1007409}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3669755
Cr-Commit-Position: refs/branch-heads/5060@{#309}
Cr-Branched-From: b83393d-refs/heads/main@{#1002911}

chrome/browser/feed/android/java/res/layout/feed_placeholder_layout.xml

@@ -7,6 +7,12 @@
     android:layout_height="wrap_content"
     android:orientation="vertical">
 
+    <View
+        android:id="@+id/divider"
+        android:layout_width="match_parent"
+        android:layout_marginBottom="@dimen/feed_divider_margin_bottom"
+        style="@style/FeedHorizontalDivider" />
+
     <LinearLayout
         android:id="@+id/feed_placeholder_header"
         android:layout_width="match_parent"