Let the background color of the previews can be adapted by different wallpapers.

Remove the hardcoded background colors and let the background colors respect the theme's android:colorBackground attribute.

PS: Need to follow the steps in go/usingmonet to let the background color can change with the wallpapers.

Screenshot:
https://screenshot.googleplex.com/3K5W5BGG2tFBxnb.png

Bug: 173549815
Change-Id: I090fa9eb2303c78959f44649cfb28d1346dcf551
diff --git a/res/layout/fragment_theme_picker.xml b/res/layout/fragment_theme_picker.xml
index 2e3a6bb..893be46 100644
--- a/res/layout/fragment_theme_picker.xml
+++ b/res/layout/fragment_theme_picker.xml
@@ -18,8 +18,7 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:orientation="vertical"
-    android:background="?android:colorPrimary">
+    android:orientation="vertical">
     <include layout="@layout/section_header"/>
 
     <FrameLayout
@@ -36,7 +35,6 @@
                 android:layout_height="0dp"
                 android:paddingTop="@dimen/preview_content_padding_top"
                 android:paddingBottom="@dimen/preview_content_padding_bottom"
-                android:background="@color/preview_pager_background"
                 android:clipToPadding="false"
                 app:layout_constrainedHeight="true"
                 app:layout_constraintStart_toStartOf="parent"
@@ -55,6 +53,7 @@
                 android:layout_height="0dp"
                 android:layout_gravity="bottom|center_horizontal"
                 android:layout_marginTop="10dp"
+                android:background="?android:colorPrimary"
                 app:layout_constraintStart_toStartOf="parent"
                 app:layout_constraintEnd_toEndOf="parent"
                 app:layout_constraintTop_toBottomOf="@+id/preview_card_container"