DO NOT MERGE Add feature flag for PixelLiveWallpaper dogfood in Settings

Add feature flag (settings_pixel_wallpaper_category_switch) in developer
option for PixelLiveWallpaper's dogfood user to turn downloadable Live
wallpaper's category on or off. Expected expiration on: 2019/07/31.

Bug: 128962566
Test: Manual check this feature flag in developer option
Change-Id: Ie9d9180605cf92cf5618999d5a76f4fd87875859