Properly compute scroll for Taskbar All Apps.

It was hooked up to Launcher All Apps scroll, which is a separate
instance from Taskbar.

Test: atest WMShellFlickerTests
Fix: 270770472
Change-Id: I14b11c8b5f1143886dd5976a12bba9e4ee66aa96

quickstep/src/com/android/launcher3/taskbar/allapps/TaskbarAllAppsController.java

@@ -135,10 +135,15 @@ public final class TaskbarAllAppsController {
         overlayContext.getDragController().setDisallowLongClick(mDisallowLongClick);
     }
 
-
     @VisibleForTesting
     public int getTaskbarAllAppsTopPadding() {
         // Allow null-pointer since this should only be null if the apps view is not showing.
         return mAppsView.getActiveRecyclerView().getClipBounds().top;
     }
+
+    @VisibleForTesting
+    public int getTaskbarAllAppsScroll() {
+        // Allow null-pointer since this should only be null if the apps view is not showing.
+        return mAppsView.getActiveRecyclerView().computeVerticalScrollOffset();
+    }
 }