Reuse widget picker's surface color in add item dialog.

In ag/28129361 add item dialog's color was changed to reference
material color token defined in launcher theme. But add item activity
inherits from Theme.Translucent.NoTitleBar for its appearance (while
launcher theme uses deviceDefault.Light), so, cannot inherit as is from
the launcher theme. Since, there is already a token specific for widget
picker surface color imported via widgetTheme, using it so it stays
in sync with main picker.

Bug: 354604590
Test: Manual
Flag: EXEMPT BUGFIX
Change-Id: I61ed5e65b0efd065490a54251028cde5888e4391
diff --git a/res/drawable/add_item_dialog_background.xml b/res/drawable/add_item_dialog_background.xml
index be4765a..39af989 100644
--- a/res/drawable/add_item_dialog_background.xml
+++ b/res/drawable/add_item_dialog_background.xml
@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <shape xmlns:android="http://schemas.android.com/apk/res/android"
     android:shape="rectangle" >
-    <solid android:color="?attr/materialColorSurfaceContainerHighest" />
+    <solid android:color="?attr/widgetPickerPrimarySurfaceColor" />
     <corners
         android:topLeftRadius="?android:attr/dialogCornerRadius"
         android:topRightRadius="?android:attr/dialogCornerRadius" />