Keep alternate reference to mStatusBar and mNavigationBar for flexible insets

Needed in order to ensure the immersive confirmation dialog and behavior
work correctly and for the swipe directions to updated based on system
bar position

Bug: 156304383
Test: manual (change immersive behavior), atest WindowInsetsControllerTests
Change-Id: Id4258d06e9d02a55b46afd53eefb7ec19a50a2ed