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/widget/WidgetsContainerView.java

@@ -271,7 +271,7 @@ public class WidgetsContainerView extends BaseContainerView
 
     @Override
     public boolean supportsFlingToDelete() {
-        return false;
+        return true;
     }
 
     @Override