Don't allow addChildSessionId after session is sealed.

To be consistent with the documented behavior of sealed sessions:

  "Once sealed, no further changes may be made to the session. A session
  is sealed the moment {@link Session#commit(IntentSender)} is called."

Bug: 123391593
Test: atest RollbackTest
Change-Id: I5d552ef1c115bd98c5a552e2b108d0ee6fbb297f