Remove hardcoded color for settings dashboard

1. Change card_background to default background color. In settings
light theme, background color is exactly the #fafafa
2. Change card_background_grey to colorSecondary, which is
material_grey_200 in settings light theme.

Bug: 36139640
Test: Screenshot
Change-Id: I20081e768985ce6c0b9000448ae48599f18b293a
diff --git a/res/layout/dashboard_category.xml b/res/layout/dashboard_category.xml
index 2e5dd5c..1314c16 100644
--- a/res/layout/dashboard_category.xml
+++ b/res/layout/dashboard_category.xml
@@ -19,8 +19,7 @@
     android:layout_width="match_parent"
     android:layout_height="@dimen/dashboard_category_height"
     android:orientation="vertical"
-    android:paddingBottom="8dip"
-    android:background="@color/card_background">
+    android:paddingBottom="8dip">
 
     <TextView android:id="@android:id/title"
         android:layout_width="match_parent"
diff --git a/res/layout/dashboard_spacer.xml b/res/layout/dashboard_spacer.xml
index 8d58c80..3ebcb09 100644
--- a/res/layout/dashboard_spacer.xml
+++ b/res/layout/dashboard_spacer.xml
@@ -18,4 +18,4 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="8dp"
-    android:background="@color/card_background_grey"/>
+    android:background="?android:attr/colorSecondary"/>
diff --git a/res/layout/support_fragment.xml b/res/layout/support_fragment.xml
index 7c79a63..6864d32 100644
--- a/res/layout/support_fragment.xml
+++ b/res/layout/support_fragment.xml
@@ -27,6 +27,5 @@
     <android.support.v7.widget.RecyclerView
         android:id="@+id/support_items"
         android:layout_height="match_parent"
-        android:layout_width="match_parent"
-        android:background="@color/card_background"/>
+        android:layout_width="match_parent"/>
 </FrameLayout>
diff --git a/res/layout/support_tile_spacer.xml b/res/layout/support_tile_spacer.xml
index 41cc221..aa86e1e 100644
--- a/res/layout/support_tile_spacer.xml
+++ b/res/layout/support_tile_spacer.xml
@@ -18,5 +18,4 @@
 <View
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
-    android:layout_height="@dimen/support_spacer_height"
-    android:background="@color/card_background"/>
+    android:layout_height="@dimen/support_spacer_height"/>