Reducing Selection to only rely on nodeIndexCalculator and not on nodeIndex

[carolynqu]

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.