[NTP Snippets] 📷 Add placeholders and start caching thumbnails

chrome/android/java/res/layout/new_tab_page_snippets_card.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2015 The Chromium Authors. All rights reserved.
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file. -->
 
 <RelativeLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/snippets_card_view"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
@@ skipping 28 matching lines @@
         android:id="@+id/article_publisher"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:layout_below="@+id/article_snippet"
         android:layout_toStartOf="@+id/article_thumbnail"
         android:layout_marginTop="16dp"
         android:maxLines="1"
         android:ellipsize="end"
         android:textSize="14sp"
         android:textColor="@color/snippets_publisher_name_color" />
-        
+
     <ImageView
         android:id="@+id/article_thumbnail"
+        android:background="@color/snippets_thumnail_placeholder_bg"
         android:layout_width="@dimen/snippets_thumbnail_size"
         android:layout_height="@dimen/snippets_thumbnail_size"
         android:layout_alignParentTop="true"
         android:layout_alignParentEnd="true"
         android:layout_marginStart="16dp"
         android:scaleType="centerCrop"
         android:contentDescription="@null"
         android:src="@null"/>
 </RelativeLayout>