Add FallbackTaskbarUIController

- Refactor TaskbarManager to accept StatefulActivity instead of BaseQuickstepLauncher
- Update the new StatefulActivity in onOverviewTargetChange()
- Cleanup LauncherTaskbarUIController a bit to avoid initializing TaskbarActivityContext, TaskbarDragLayer, and TaskbarViewController in the constructor, and instead get these from mControllers after init()

Test: No change in behavior
Bug: 188790554
Change-Id: Ic4d69d73deb6dada416876eb265db57803c3adfd