Preventing start back navigation before new activity window created.

If the new window has not yet been added to the launching activity,
skip prepare the pending animation, as the final state after the
transition finish is unpredictable.

Flag: EXEMPT bugfix
Bug: 391999788
Test: presubmit
Change-Id: I36cb1b69a5a3f4a2d438f962bb7792cc7155baa9