Remove the wallpaper task and tasks below from
`TopTaskTracker.getCachedTopTask` thus the empty task can be
really empty.
Also update the usage of the cached top task to avoid crash
because it can be empty now.
Bug: 402218420
Flag: EXEMPT refactor
Test: m
Change-Id: I15835ec3bf84b9ea5b6b2aba731f9f5b8a17353f