Respect CentralSurfaces being optional in NavBarHelper

CentralSurfaces is an optional element in SystemUI so some extensions
may not include this. Therefore, it needs to be checked if it is present
before use.

Bug: 241105832
Test: atest CarActivityServiceTaskMonitorUnitTest --rerun-until-failure
20, atest com.android.systemui.navigationbar

Change-Id: I32f66562af699e16a0a5deb3937c002f9fa2ef09