Only setup SecondaryDropTarget in onDragStart

- In LauncherAccessibilityDelegate.getsupportedActions, it calls supportsAccessibilityDrop on each button regardless if DropTargetBar is visible or not. SecondaryDropTarget.supportsAccessibilityDrop attempts to setupUi which causes measure/layout of DropTargetBar (as well as DragLayer) each time LauncherAccessibilityDelegate.getsupportedActions is called
- To avoid this, only setupUi in onDragStart. In other cases don't call setUi.
- Also fixed a bug that when accessibility is on, upon dragging a suggested app, the secondary action suddenly changed to Uninstall

Fix: 233383721
Test: Enable accessibility, observe no excessive amount of onLayout is called on DropTargetBar
Change-Id: I4d1a00d320cceed81fa1312b5fc5f1081637bea8

src/com/android/launcher3/ButtonDropTarget.java

@@ -179,7 +179,12 @@ public abstract class ButtonDropTarget extends TextView
 
     @Override
     public void onDragStart(DropTarget.DragObject dragObject, DragOptions options) {
-        mActive = !options.isKeyboardDrag && supportsDrop(dragObject.dragInfo);
+        if (options.isKeyboardDrag) {
+            mActive = false;
+        } else {
+            setupItemInfo(dragObject.dragInfo);
+            mActive = supportsDrop(dragObject.dragInfo);
+        }
         setVisibility(mActive ? View.VISIBLE : View.GONE);
 
         mAccessibleDrag = options.isAccessibleDrag;
@@ -191,6 +196,11 @@ public abstract class ButtonDropTarget extends TextView
         return supportsDrop(dragObject.dragInfo);
     }
 
+    /**
+     * Setups button for the specified ItemInfo.
+     */
+    protected abstract void setupItemInfo(ItemInfo info);
+
     protected abstract boolean supportsDrop(ItemInfo info);
 
     public abstract boolean supportsAccessibilityDrop(ItemInfo info, View view);