Execute app transition when start locking task

This issue always existed but now only became visible because
occluding/unoccluding Keyguard relies on activity transitions.

Test: Have pattern, lock task, unlock without 5 seconds, make sure
activity is not lingering around.
Bug: 32771855

Change-Id: Ib75bf904dd69db64bd64d13d518897ce9cf44394

services/core/java/com/android/server/am/ActivityStackSupervisor.java

@@ -3779,6 +3779,7 @@ public class ActivityStackSupervisor extends ConfigurationContainer
             findTaskToMoveToFrontLocked(task, 0, null, reason,
                     lockTaskModeState != LOCK_TASK_MODE_NONE);
             resumeFocusedStackTopActivityLocked();
+            mWindowManager.executeAppTransition();
         } else if (lockTaskModeState != LOCK_TASK_MODE_NONE) {
             handleNonResizableTaskIfNeeded(task, INVALID_STACK_ID, task.getStackId(),
                     true /* forceNonResizable */);