Fix live wallpaper not showing on lock screen after restore

A recent change in nyc-mr2 means that setWallpaperComponent will
promote any static system wallpaper to the lock screen.  If we don't
actually have a static lock wallpaper to restore, we need to explicitly
clear the lock wallpaper to have the live one show through

Test: manually using adb backup / restore
Bug: 35090402
Change-Id: I8a2c5ffbaefe172c9d78a9f746ab87d9bc6b7448