Fix WallpaperManager APIs

When support for lockscreen wallpapers was added in API level 24 the
behaviour for earlier API versions changed. Calls to the old 'set' APIs
no longer affect the overall device wallpaper, and can result in an end
user not being able to change their lockscreen wallpaper.

This upload restores the original API behaviour.

Bug: 31204228
Change-Id: Ia16d2e2e379c54d798eef8f5c653099c2c581d78