Only skip hierarchy op which may reorder task in task lock mode

Otherwise when entering task lock mode from split-screen, the
request from WindowManagerProxy#applyDismissSplit is ignored.
That causes launch root to remain and affect new launch tasks.

Bug: 186764983
Test: MultiWindowTests#testDisallowHierarchyOperationWhenInLockedTask
Test: 1. Enable App pinning
      2. Enter split-screen mode
      3. Pin the task on secondary-split
      4. Unpin the task and launch other new task
      The new task should fill screen bounds.
      "adb shell dumpsys activity a | grep mLaunchRootTasks"
      must be empty.

Change-Id: I890a9e1234b11772f4183168e085b75d31859e20