Fix for FAB targets not matching current user on user switch

On user switch, waits for A11yManagerService to finish initializing user before showing the FAB.
This makes sure the FAB loads with the correct information.

Demonstration video: https://x20web.corp.google.com/users/jo/jonesriley/fabFootage/fabUserSwitchFix.mp4

Flag: NONE Internal change
Test: Switch between users with different FAB targets (>0 targets). On user switch, the FAB should display the correct targets.
Bug: 360249982
Change-Id: I434161d0add3e87b6d21a6590bdf44a2b113245c