Move PreviewPager related code from ThemePicker/ to WallPaperPicker2/.

Since there's a view pager in the new UI design for wallpaper picker.

Bug: 149374062
Test:
. build/envsetup.sh
tapas WallpaperPicker2 (and ThemePicker/WallpaperPickerGoogleDebug/WallpaperPickerGoogleRelease)
make -j40

Change-Id: I78e12419fb7008653db185d1d341e8659a8764de
diff --git a/res/layout/fragment_theme_picker.xml b/res/layout/fragment_theme_picker.xml
index 42c9817..1de5a3c 100644
--- a/res/layout/fragment_theme_picker.xml
+++ b/res/layout/fragment_theme_picker.xml
@@ -32,7 +32,7 @@
             android:layout_width="match_parent"
             android:layout_height="match_parent">
 
-            <com.android.customization.widget.PreviewPager
+            <com.android.wallpaper.widget.PreviewPager
                 android:id="@+id/theme_preview_pager"
                 android:layout_width="match_parent"
                 android:layout_height="wrap_content"