Update back-gesture-allowed state before calling state change callback

- When a device is first set up, isHandlingGestures() is false because
  we are in SUW, but when user setup completes, we need to update
  mIsBackGestureAllowed prior to calling the callback which will
  query that state, otherwise there is no other signal to trigger
  another relayout after the state is set.

Bug: 331601358
Test: atest Launcher3Tests
Change-Id: I9b37df4b3e63c6fcdd91e166c613866f8f046554