Clear lock bitmaps when wallpaper applied to home+lock

Let's never keep old bitmap in files. Otherwise if a user has a live
wallpaper on lock screen only, and backups & restore to an older device,
the old bitmap will reappear. Also otherwise, getWallpaperFile() may
return an old bitmap even if the user has a lockscreen live wallpaper.

Only effective with the lockscreen lwp flag

Flag: lockscreen live wallpaper
Bug: 283091821
Test: atest WallpaperManagerTest
Change-Id: I8cd9af52e1a5f3a0388b3af22ab6c8c441520a8e