Uses grey600(#757575) for bottom bar to pass A11y check am: fb292ac0ac am: b2c8dd1c8d

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/WallpaperPicker2/+/11689622

Change-Id: I716224aee65015cf586a20788148bccf71c54496