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/WorkspaceLayoutManager.java

@@ -32,6 +32,8 @@ public interface WorkspaceLayoutManager {
     int EXTRA_EMPTY_SCREEN_ID = -201;
     // The is the first screen. It is always present, even if its empty.
     int FIRST_SCREEN_ID = 0;
+    // This is the second page. On two panel home it is always present, even if its empty.
+    int SECOND_SCREEN_ID = 1;
 
     /**
      * At bind time, we use the rank (screenId) to compute x and y for hotseat items.