Reorganize taskbar controllers

Organize existing properties as follows:
- TaskbarViewController contains properties affecting TaskbarView (though child icons are still supplied by TaskbarHotseatController)
- TaskbarDragLayerController contains properties related to TaskbarDragLayer itself
- Renamed NavbarButtonUiController to NavbarButtonsViewController, following the pattern of TaskbarViewController and TaskbarDragLayerController
- TaskbarControllers contains the different controllers to make it easier to construct, initialize, destroy, and pass them around
- Removed TaskbarIconController as its responsibilities were moved to more specific controllers

Test: compiles and runs, manually tested
Bug: 187353581
Change-Id: Idccd95d47117101bf9617e5532a5b87635d2b8f6