Cleanup taskbar touchableInsets

- Remove obsolete visibleInsets hack and isDraggingItem() check
- Have TaskbarController determine whether taskbar is touchable,
  rather than TaskbarContainerView.
- Disallow touches during transition to home (can potentially
  improve this later)

Test: Swupe home, tap taskbar folder during transition, ensure
don't end up in broken state with the uninteractable folder open
Bug: 182512211

Change-Id: Id625ce8b9b876a8a1b450b7d6fb771540e23bc08