Fixes blank lock screen wallpaper after change wp from home to both

Repro steps:
* Set lock to wallpaper A
* Set home to wallpaper B
* Set home+lock to same wallpaper B

Bug: 253507223
Test: manually verified that changing home->both does not produce blank
Change-Id: Icdc35fa8ffb4769173be45310a56d59f008fbc5f