Persist wallpaper changes after migration

After a migration (e.g. set a wallpaper on home when having a live
wallpaper on home+lock), the changes need to be persisted in
wallpaper_info.xml. Otherwise after reboot, the wallpapers won't be
rendered correctly.

The complete() method of this CL is only called when the lockscreen
livewallpaper flag is enabled, so this change is under flag.

Flag: lockscreen lwp
Bug: 286061806
Test: atest WallpaperManagerTest
Test: manually check that the bug is fixed
Change-Id: I704761c20e66e279a0e080e29dff3fa788d2ecf1