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.

从 packages/devtools_app/lib/src/shared/table.dart 开始，其中 Selection 目前包含 nodeIndex 和 nodeIndexCalculator。然后检查 memory_heap_tree 中 nodeIndex 的每一处变更，并追踪这些更新如何迁移到 nodeIndexCalculator。完成的标准是 Selection 不再依赖 nodeIndex，并且 memory heap tree 一致地使用 calculator。

由索引模型根据 Issue 内容生成。