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