Add bold effect to the selected button of wallpaper preview am: e14acb0a5d am: 9a5ac69a20

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/WallpaperPicker2/+/11751398

Change-Id: I8001c627ee4b5713ad02e1d67958122aa6cb0fb7