Add default placeholder when a dream doesn't provide a preview image.

Bug: 213887094
Test: locally on device
Change-Id: Ie4b96eadb0889a5216cda1cfe1cf0cc17a2ad652

res/drawable/dream_default_preview_icon.xml

@@ -0,0 +1,24 @@
+<!--
+  ~ Copyright (C) 2022 The Android Open Source Project
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~      http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+        android:width="24dp"
+        android:height="24dp"
+        android:viewportWidth="24"
+        android:viewportHeight="24">
+    <path android:fillColor="?android:attr/textColorSecondary"
+          android:pathData="M3,19Q2.175,19 1.588,18.413Q1,17.825 1,17V7Q1,6.175 1.588,5.588Q2.175,5 3,5H13Q13.825,5 14.413,5.588Q15,6.175 15,7V17Q15,17.825 14.413,18.413Q13.825,19 13,19ZM3,17H13Q13,17 13,17Q13,17 13,17V7Q13,7 13,7Q13,7 13,7H3Q3,7 3,7Q3,7 3,7V17Q3,17 3,17Q3,17 3,17ZM17,19V5H19V19ZM21,19V5H23V19ZM4,15H12L9.4,11.5L7.5,14L6.1,12.15ZM3,7Q3,7 3,7Q3,7 3,7V17Q3,17 3,17Q3,17 3,17Q3,17 3,17Q3,17 3,17V7Q3,7 3,7Q3,7 3,7Z"/>
+</vector>

res/drawable/dream_preview_rounded_bg.xml

@@ -15,6 +15,8 @@
   -->
 
 <shape xmlns:android="http://schemas.android.com/apk/res/android"
+       xmlns:androidprv="http://schemas.android.com/apk/prv/res/android"
        android:shape="rectangle">
+    <solid android:color="?androidprv:attr/colorSurfaceHighlight" />
     <corners android:radius="@dimen/dream_item_corner_radius"/>
 </shape>