Fix "appearance glitch" in Back Arrow

This avoids playing the Cancellation animation when we receive an
ACTION_CANCEL (and just goes straight to GONE), which fixes the glitch.

Bug: 237442123
Test: manual
Change-Id: Iab0fb98f5a71e1fe80d3949b4fbb35055c731b3f

packages/SystemUI/src/com/android/systemui/navigationbar/gestural/BackPanelController.kt

@@ -321,7 +321,10 @@ class BackPanelController private constructor(
                 velocityTracker = null
             }
             MotionEvent.ACTION_CANCEL -> {
-                updateArrowState(GestureState.CANCELLED)
+                // Receiving a CANCEL implies that something else intercepted
+                // the gesture, i.e., the user did not cancel their gesture.
+                // Therefore, disappear immediately, with minimum fanfare.
+                updateArrowState(GestureState.GONE)
                 velocityTracker = null
             }
         }