Fixing crash when task is already parent of activity.

- When launching an activity into a task, it's parent is already set
  which can cause the reparent call to crash.  Instead, just move the
  activity to the top.

Bug: 34933399
Test: Launch an app that doesn't support multiwindow, and engage split
      screen, or launch a PiP activity and tap to show the menu

Change-Id: Iaa36003e88f4021f2483dedc71802023369d6558