[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 "492dac69".
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