Fix NPE in edge back gesture handler

- TaskBarDelegate creates an EdgeBackGestureHandler, but does not set
  a stage change callback (not needed currently)

Fixes: 197831135
Test: atest SystemUITests

Signed-off-by: Winson Chung <winsonc@google.com>
Change-Id: I35841c077506d7dad35b0c53a72d9907c205cb52

packages/SystemUI/src/com/android/systemui/navigationbar/gestural/EdgeBackGestureHandler.java

@@ -384,7 +384,7 @@ public class EdgeBackGestureHandler extends CurrentUserTracker
     private void onNavigationSettingsChanged() {
         boolean wasBackAllowed = isHandlingGestures();
         updateCurrentUserResources();
-        if (wasBackAllowed != isHandlingGestures()) {
+        if (mStateChangeCallback != null && wasBackAllowed != isHandlingGestures()) {
             mStateChangeCallback.run();
         }
     }