Fix random systemui crashes during cts test DO NOT MERGE

    systemui may crash due
    to an uninitialized value of mNavigationBar probably because
    of some race condition during initialization caused by emulation
    performance issues:

    11-01 19:48:58.634  5064  5064 E AndroidRuntime: FATAL EXCEPTION: main
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: Process: com.android.systemui, PID: 5064
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.android.systemui.statusbar.phone.LightBarTransitionsController.dump (java.io.FileDescriptor, java.io.PrintWriter, java.lang.String[])' on a null object reference
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at com.android.systemui.statusbar.phone.LightBarController.dump(LightBarController.java:241)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at com.android.systemui.statusbar.phone.StatusBar.dump(StatusBar.java:3448)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at com.android.systemui.SystemBars.dump(SystemBars.java:48)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at com.android.systemui.SystemUIService.dump(SystemUIService.java:52)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at android.app.ActivityThread.handleDumpService(ActivityThread.java:3488)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at android.app.ActivityThread.-wrap9(Unknown Source:0)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at android.app.ActivityThread.handleMessage(ActivityThread.java:1726)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:105)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at android.os.Looper.loop(Looper.java:164)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:6541)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at com.android.internal.os.Zygote.run(Zygote.java:240)
    11-01 19:48:58.634  5064  5064 E AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

    This patches adds an explicit checks for (mNavigationBar != null)
    everywhere it is used.

    Test: cts test.

Change-Id: I8056aef54a10bbb1970bc2ce4913e7a618498fd3