[WM] Don't moveTaskToBackInner if it's already detached
The root cause is moveTaskToBackInner may start at a later time,
should ensure that the task is still attached before calling
moveTaskToBackInner.
Similar code was applied at "492dac69a31a67878b8c7bbd97a0227f82b7fc2b".
However, it cannot cover if a child task is removed in a root task
with deferred transition.
Bug: 372834400
Test: Automation Test in SetupWizard (Root - Child Task)
Change-Id: I9341fd5edda879e33c42865df279440591546b54
1 file changed