Remove the empty Task when failed starting it from Recents

The recent task was restored and added to WM hierarchy for the
activity launch. However, the activity launch was aborted, which
leaves an empty Task in the WM.

Remove the Task if there's no child and should be removed when empty.
Note that the Task would also be removed from the Recents because
the Task hasn't been visible, see Task#shouldAutoRemoveFromRecents.

Bug: 357967390
Test: repro steps on the bug
Flag: EXEMPT bugfix
Change-Id: If1e4b924fa0c5dc47209be1e08e8409ed3a0b056