Clear child fragment manager after it is destroyed.

Bug 26945643

After a child fragment manager has been destroyed, it should not
be reused because it is no longer attached to the host. Clearing
it ensures that it will be recreated when it is needed again when
the fragment has been reattached.

Change-Id: I7e52a42a66d761ab69596fdcdb5b2b2e3b4dcc6c

core/java/android/app/Fragment.java

@@ -2509,6 +2509,7 @@ public class Fragment implements ComponentCallbacks2, OnCreateContextMenuListene
             throw new SuperNotCalledException("Fragment " + this
                     + " did not call through to super.onDestroy()");
         }
+        mChildFragmentManager = null;
     }
 
     private static Transition loadTransition(Context context, TypedArray typedArray,