Refactor getHighestVisibleTaskIndex to return TaksView and remove requiredTaskViewAt

- Removed last usage of requiredTaskViewAt, which is a dangerous method assuming there must be a TaskView at certain index

Bug: 379942019
Test: manually on Task rebalancing after dismiss
Flag: EXEMPT refactor
Change-Id: Ib53e0f47435d6fb9f8fd42f89b045160e75b5e54
1 file changed