AppTransition has 4 states:
private final static int APP_STATE_IDLE = 0;
private final static int APP_STATE_READY = 1;
private final static int APP_STATE_RUNNING = 2;
private final static int APP_STATE_TIMEOUT = 3;
AppTransition#setReady
one case is when resumed
ActivityRecord#completeResumeLocked
WindowSurfacePlacer#requestTraversal to
performSurfacePlacementNoTrace to
RootWindowContainer will call checkAppTransitionReady, which wil check if ready ,i it will call
AppTransitionController#handleAppTransitionReady()