Remove SceneTransitionLayoutImpl.readyScenes

This CL removes STLImpl.readyScenes, now that MovableElement does not
rely on it for correctness anymore. This means that transitions are not
delayed by one frame and that STLImpl.isTransitionReady does not need to
be checked anymore.

Bug: 316901148
Test: Existing STL tests
Flag: N/A
Change-Id: I304f4327956282e79539b3dca574950b7ae0d1d6
3 files changed