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/DragSource.java

@@ -37,7 +37,7 @@ public interface DragSource {
 
     /**
      * @return whether items dragged from this source supports 'Delete' drop target (e.g. to remove
-     * a shortcut.
+     * a shortcut.) If this returns false, the drop target will say "Cancel" instead of "Remove."
      */
     boolean supportsDeleteDropTarget();