Reducing Selection to only rely on nodeIndexCalculator and not on nodeIndex

**Currently**
Selection class in table.dart contains both nodeIndex and nodeIndexCalculator. https://github.com/flutter/devtools/blob/master/packages/devtools_app/lib/src/shared/table.dart#L321

**Future**
Only NodeIndexCalculator is needed and all changes to nodeIndex can be migrated to nodeIndexCalculator. Only changes of nodeIndex is found in memory_heap_tree.

Start in packages/devtools_app/lib/src/shared/table.dart, where Selection currently contains nodeIndex and nodeIndexCalculator. Then inspect memory_heap_tree for every nodeIndex change and trace how those updates can move to nodeIndexCalculator. Done means Selection no longer relies on nodeIndex and the memory heap tree uses the calculator consistently.

Written by the indexing model from the issue text.