Fix not tracking splitting tasks correctly

When drag-to-repace one of the splitting tasks, it might reports to
TopTaskTracker the original task changed to undefined after repoting the
new task appeared in a split pair. Result to clear the split tasks
record unexpectedly in TopTaskTracker. This makes sure to clear the
correct stage and prevent override the correct split record.

Fix: 277982233
Test: manual check the behavior and verify the reocrd in TopTaskTracker
Change-Id: Ifce2efdde6d99508f1afa844b3ad424071e13a8f