[Status Bar Refactor] Improvements to StatusBarIconList:

1. Instead of StatusBarIconControllerImpl extending StatusBarIconList,
   just have the icon list as a private member variable.
2. Stop exposing slot indices from StatusBarIconList. External clients
   can always use the slot name instead, so we can keep the indices
   private.

Test: atest StatusBarIconListTest
Bug: 237533036
Change-Id: I811481558ba16f475bdff36c1b1b2b0501aa1216