[CD Taskbar] Refactor TaskbarManager to store TaskbarRootLayouts in a map

Refactor the TaskbarManager's existing usages of TaskbarRootLayouts to use a map, so that there can be a 1:1 relationship between TaskbarRootLayouts and TaskbarActivityContexts.

Flag: EXEMPT not adding new behavior
Bug: 380351840
Test: Manual
Change-Id: Ib4379a9b6d50b8bdd64b70d7b15ce798ebb42731