Allow split with an existing split task

- Determine the task in the group being clicked in split-select mode
  and launch split using that selected task
- Also make sure we don't handle drag cancel twice when we are animating
  the return of the drag surface

Bug: 219060441
Test: Split from home/overview with fullscreen+fullscreen task, and with
      fullscreen+split task

Change-Id: I48ec0a82812197803ff4b3698830a9cb705719e3

quickstep/src/com/android/launcher3/taskbar/TaskbarDragController.java

@@ -469,13 +469,19 @@ public class TaskbarDragController extends DragController<TaskbarActivityContext
             }
         });
         mReturnAnimator.addListener(new AnimatorListenerAdapter() {
+            private boolean mCanceled = false;
+
             @Override
             public void onAnimationCancel(Animator animation) {
                 cleanUpSurface();
+                mCanceled = true;
             }
 
             @Override
             public void onAnimationEnd(Animator animation) {
+                if (mCanceled) {
+                    return;
+                }
                 cleanUpSurface();
             }