Add a BUILD file for the WallpaperPicker app. am: 55686c886d am: 64a86bd585 am: 24573416f9

Original change: https://android-review.googlesource.com/c/platform/packages/apps/WallpaperPicker/+/1833217

Change-Id: I7e369f4cc74c195ee4cacd2031eedd1a88d0c024