Interrupted predictive back does not call canChangeScene

This CL ensures that STLState.canChangeScene() is only called right
before the actual STLState.transitionState.currentScene is going to be
changed. In particular, it should not be called if the predictive back
transition is interrupted by another transition.

Bug: 350705972
Test: PredictiveBackHandlerTest
Flag: com.android.systemui.scene_container
Change-Id: I79fd72609d78982b17caa5303154d4e40e3d7ae4