Added 'Cancel' drop target from all apps and widget picker.

- Reuse DeleteDropTarget since it's the same effect, but with "Cancel"
  instead of "Remove" if supportsDeleteDropTarget() returns false.
- Rename related strings (but not their values)

Bug: 24104015
Bug: 24099531
Change-Id: Ia9fbcaa17bb17f7aa31df1f830298da01544c178

src/com/android/launcher3/BaseDropTargetBar.java

@@ -101,7 +101,7 @@ public abstract class BaseDropTargetBar extends FrameLayout implements DragContr
      */
     @Override
     public void onDragStart(DragSource source, ItemInfo info, int dragAction) {
-        showDropTarget();
+        showDropTargets();
     }
 
     /**
@@ -115,15 +115,15 @@ public abstract class BaseDropTargetBar extends FrameLayout implements DragContr
     @Override
     public void onDragEnd() {
         if (!mDeferOnDragEnd) {
-            hideDropTarget();
+            hideDropTargets();
         } else {
             mDeferOnDragEnd = false;
         }
     }
 
-    public abstract void showDropTarget();
+    public abstract void showDropTargets();
 
-    public abstract void hideDropTarget();
+    public abstract void hideDropTargets();
 
     public abstract void enableAccessibleDrag(boolean enable);