Clean up PreviewFragment and its child classes (Part 1) am: ecfd6eb254 am: 9527176c6e

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/WallpaperPicker2/+/11627242

Change-Id: I064a2905503811d6b235b7666fbb585d33ce161f