Fix wallpaper input consumer handling in InputMonitor

Only add the wallpaper input consumer if the wallpaper is visible.
Otherwise it breaks Setup Wizard.
If there is no wallpaper at all, still add the consumer at the end.

Bug: 28026545
Change-Id: I833f59ef51f1b151fc110f204aa666398fa764f9
(cherry picked from commit e0c1761c7695929d442d051345d3d3152cd3b103)