Remove hard-coded windowBackground to used that of the theme

Bug: 80446342
Test: In head unit
Change-Id: I519af7c0e8c905a7d8f281a9ef29c9525a706acf
diff --git a/library/res/layout/car_setup_wizard_toolbar.xml b/library/res/layout/car_setup_wizard_toolbar.xml
index 6431acd..668e95d 100644
--- a/library/res/layout/car_setup_wizard_toolbar.xml
+++ b/library/res/layout/car_setup_wizard_toolbar.xml
@@ -21,7 +21,7 @@
 <!-- Note: This layout needs a background in order for elevation to show up. -->
 <RelativeLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
-    android:background="@color/windowBackground"
+    android:background="?attr/android:windowBackground"
     android:layout_width="match_parent"
     android:layout_height="@dimen/car_app_bar_height">
 
diff --git a/library/res/values/colors.xml b/library/res/values/colors.xml
index 0e2d71d..b0aff07 100644
--- a/library/res/values/colors.xml
+++ b/library/res/values/colors.xml
@@ -15,9 +15,6 @@
     limitations under the License.
 -->
 <resources>
-    <!-- The default color for all activities in the SetupWizard. -->
-    <color name="windowBackground">@color/car_grey_50</color>
-
     <!-- The color of the status bar. -->
     <color name="colorPrimaryDark">@color/car_grey_300</color>
 </resources>