Fix double status bar icons

Don't allow the status bar icon slot list to be changed because
it confuses the relationship between StatusBarIconList and
StatusBarIconController.  Set it in the constructor to enforce
this.

Change-Id: Ieeea0a9efad88179d1cccc0e5702899333de2e72
Fixes: 28524184
(cherry picked from commit c6fe61c5)