Destroy activity if task is removed directly

Otherwise the activity is unreachable from hierarchy but
WindowProcessController still references to the activity.

- Task#removeImmediately will go super.removeImmediately so
  its children's removeImmediately will also be called. That
  is the place to destroy activity.
- Avoid reentrant of Task#removeImmediately because it is
  confusing to see multiple wm_task_removed with different
  reasons, e.g. [deleteRootTask, setTaskOrganizer] and
  [removeTask, cleanUpResourcesForDestroy]. And it also
  eliminates duplicated invocation of super.removeImmediately.
- Add validation for setLaunchRoot.

Bug: 177633638
Test: ActivityRecordTests#testRemoveImmediately

Change-Id: I762f0a720d1709bb72deaacd857d5e79ce48d1b2