Two panel home empty page removing logic change

- Don't remove a single page if it's empty, only if both pages are empty.
- Add back empty pages after they were removed while the phone
  was on single panel home.
- On two panel home don't add new workspace items onto the second screen

Test: manual
Bug: 196376162
Change-Id: I4c54ffa3b359a236deb3ab67adb54111e77ec896

src/com/android/launcher3/util/IntArray.java

@@ -296,7 +296,7 @@ public class IntArray implements Cloneable, Iterable<Integer> {
 
         @Override
         public void remove() {
-            throw new UnsupportedOperationException();
+            removeIndex(--mNextIndex);
         }
     }
 }