Merge "Fix system crash in Atms.notifyKeyguardFlagsChanged."

                // We might change the visibilities here, so prepare an empty app transition which

                // might be overridden later if we actually change visibilities.

                final DisplayWindowController dwc = mStackSupervisor.getActivityDisplay(displayId)

                        .getWindowContainerController();

                final ActivityDisplay activityDisplay =

                        mStackSupervisor.getActivityDisplay(displayId);

                if (activityDisplay == null) {

                    return;

                }

                final DisplayWindowController dwc = activityDisplay.getWindowContainerController();

                final boolean wasTransitionSet = dwc.getPendingAppTransition() != TRANSIT_NONE;

                if (!wasTransitionSet) {

                    dwc.prepareAppTransition(TRANSIT_NONE, false /* alwaysKeepCurrent */);