Fetch transition specs if the state is already ready

The timing to invoke override pending transition may be later
than state ready. This change makes a chance to fetch specs
when setting the override transition. So the the transition
execution can wait until the specs is fetched and apply the
correct transition type.

Bug: 35633644
Bug: 123172960
Test: manual - Enter split-screen mode and check the
      transition should be aspect scaling instead of
      normal launch animation.

Change-Id: I1072f60cb9b323c26b342bce1d0ad458d30fb5ef