Add getNavigationBarWindow override to CarStatusBar

This fixes the CarStatusBar to work with new code in the framework which
relies on getting the status bar window.  Prior to this, the base class
function was called, but returned null since the base classes' objects
are not used by the derived class.

Test:  Run systemUi on Mojave
Bug: 62948289
Change-Id: Ic3357a7bb61f1a4f4fdfd626d91f611bcad9893e