Removed StageListenerImpl and moved data into StageTaskListener

* No logical changes, did split onStatusChanged() callback into two
more granular callbacks for child count and visibility changed
* StageTaskListener holds the data for each stage and calls back into
StageCoordinator when necessary
* This will help keeping one-off classes for the stages in an effort
to generify the stages from each other.
* Also removed tests for non-shell transition paths

Bug: 349828130
Flag: EXEMPT refactor
Test: Existing tests pass, added a new test for root task visibility
Played with splitscreen on device, works as before

Change-Id: Ib1aa0bf33dc3dd641d8eb7dc9d31107eec6f62ed