Fix taskbar visibility when default-to-desktop

This CL fixes the taskbar visibility when showLockedTaskbarOnHome is
enabled.
Ife336ebd57eb2f60c2bc33ed6a4527c42111808f wrongly assumes that the
taskbar is always invisible when the launcher is visible, so this CL
fixes the assumption.

Bug: 379870867
Bug: 379785901
Flag: EXEMPT bug fix
Test: manual
Change-Id: I9043898fb499520954ee3c5463ab28208d374875