Remove onStageHasChildrenChanged() from StageCoordinator

* sendStatusChanged() was only being called from onTaskAppeared()
for when the root task appears for a given stage.
* sendStatusChanged() had two callbacks, one if the children in a stage
changed and one if the visibility of a stage's root task changed.
The former is not affected by onTaskAppeared(), so we can directly call
the visibility callback instead of going through sendStatusChanged()
* This means that the other callback for children changes is no longer
being used, so we remove that one altogether

Bug: 349828130
Flag: EXEMPT refactor
Test: Split works as expected
Change-Id: Ie7cc642bdc761b7e916e53354ac21552e77a5de8