Fix crash from resetting shortcuts to none

Wallpaper picker crashes from resetting both shortcuts to none, and
behaves incorrectly when resetting one shortcut to none, because slots
without a selection were not accounted for in the snapshot restorer.

Flag: NONE
Bug: 305156127
Test: Manually verified different shortcut reset scenarios
Change-Id: Ib218dccf3f34509b64662eafdf7ac301884ec066