Taskbar drag starts internal pre-drag before system drag

- TaskbarDragController now extends DragController.
- Currently there is no pre-drag condition, so we immediately get onDragStart(), which starts the system global drag (which cancels the original internal drag).
- Make the original view invisible during the drag and drop operation, across both internal and system drag events.
- No longer handle onDragEvent() in TaskbarView, as TaskbarDragController handles all of it now.

Test: Drag and drop from taskbar still works (bonus: starts from the correct registration point that you touched down on). Locally added a PreDragCondition and verified a seamless handoff to system drag and drop when the pre drag end condition was met.
Bug: 182981908
Change-Id: I6bf48141a5eedfc6db6f461258e880ef8146e733

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

@@ -62,10 +62,11 @@ public class TaskbarIconController {
 
         ButtonProvider buttonProvider = new ButtonProvider(mActivity);
         mImeBarView.init(buttonProvider);
-        mTaskbarView.construct(clickListener, longClickListener, buttonProvider);
+        mTaskbarView.init(new TaskbarViewCallbacks(), clickListener, longClickListener,
+                buttonProvider);
         mTaskbarView.getLayoutParams().height = mActivity.getDeviceProfile().taskbarSize;
 
-        mDragLayer.init(new Callbacks(), mTaskbarView);
+        mDragLayer.init(new TaskbarDragLayerCallbacks(), mTaskbarView);
     }
 
     public void onDestroy() {
@@ -102,7 +103,7 @@ public class TaskbarIconController {
     /**
      * Callbacks for {@link TaskbarDragLayer} to interact with the icon controller
      */
-    public class Callbacks {
+    public class TaskbarDragLayerCallbacks {
 
         /**
          * Called to update the touchable insets
@@ -160,4 +161,16 @@ public class TaskbarIconController {
             mImeBarView.setVisibility(alpha == 0 ? GONE : VISIBLE);
         }
     }
+
+    /**
+     * Callbacks for {@link TaskbarView} to interact with the icon controller
+     */
+    public class TaskbarViewCallbacks {
+        /**
+         * Returns whether no other controller is currently handling the given View's visibility.
+         */
+        public boolean canUpdateViewVisibility(View child) {
+            return !mActivity.getDragController().isDraggingView(child);
+        }
+    }
 }