Adjust window focusable by view controller

Some adjustments to note:
1. remove previous focusability changes per view controller
2. disallow calling insetsController.show/hide without window focus
3. update tests to depend on window focus
4. update DisplaySystemBarsController to refer to parent class for ime
insets

Bug: 163135884
Test: manual, atest :carsysui-presubmit
Change-Id: If5adf599bc2c676ad296f89566534c1fdc9f2492