Fix DocumentsUI animation regression.

Various fragments need backgrounds to protect their contents during
animation transitions.

Bug: 22232368
Change-Id: I4ce4c57650f9b64b3c6101d829dd484417585410
diff --git a/packages/DocumentsUI/res/layout-sw720dp/activity.xml b/packages/DocumentsUI/res/layout-sw720dp/activity.xml
index 2a273f4..221de13 100644
--- a/packages/DocumentsUI/res/layout-sw720dp/activity.xml
+++ b/packages/DocumentsUI/res/layout-sw720dp/activity.xml
@@ -55,6 +55,7 @@
             android:layout_height="match_parent"
             android:layout_weight="1"
             android:orientation="vertical"
+            android:background="@color/material_grey_50"
             android:elevation="8dp">
 
             <com.android.documentsui.DirectoryContainerView
@@ -67,6 +68,7 @@
                 android:id="@+id/container_save"
                 android:layout_width="match_parent"
                 android:layout_height="wrap_content"
+                android:background="@color/material_grey_50"
                 android:elevation="8dp" />
 
         </LinearLayout>
diff --git a/packages/DocumentsUI/res/layout/activity.xml b/packages/DocumentsUI/res/layout/activity.xml
index 43fdaf2..32431e3 100644
--- a/packages/DocumentsUI/res/layout/activity.xml
+++ b/packages/DocumentsUI/res/layout/activity.xml
@@ -51,6 +51,7 @@
             android:id="@+id/container_save"
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
+            android:background="@color/material_grey_50"
             android:elevation="8dp" />
 
     </LinearLayout>
diff --git a/packages/DocumentsUI/res/layout/fragment_directory.xml b/packages/DocumentsUI/res/layout/fragment_directory.xml
index 2c08e72..4717839 100644
--- a/packages/DocumentsUI/res/layout/fragment_directory.xml
+++ b/packages/DocumentsUI/res/layout/fragment_directory.xml
@@ -16,7 +16,8 @@
 
 <com.android.documentsui.DirectoryView xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
-    android:layout_height="match_parent">
+    android:layout_height="match_parent"
+    android:background="@color/material_grey_50">
 
     <TextView
         android:id="@android:id/empty"
diff --git a/packages/DocumentsUI/res/values/colors.xml b/packages/DocumentsUI/res/values/colors.xml
index 2b839d4..ba8d69f 100644
--- a/packages/DocumentsUI/res/values/colors.xml
+++ b/packages/DocumentsUI/res/values/colors.xml
@@ -15,6 +15,7 @@
 -->
 
 <resources>
+    <color name="material_grey_50">#fffafafa</color>
     <color name="material_grey_300">#ffeeeeee</color>
 
     <color name="item_doc_grid_background">@color/material_grey_300</color>