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

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

Change-Id: I59d8cfa8e9bbd40bfa8e6e6c959fbd57c4983aa3