When the user select a wallpaper and then click back key in SUW, it should become unselect state instead of going back to previous page. am: f24418e7b6 am: 9735bcadaa

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/WallpaperPicker2/+/11436727

Change-Id: Iee1c1176dbda4c30d429005a4710cc897c96b521