Improve usability of memory screen.

[polina-c]

Survey: go/flutter-devtools-survey-2022

The usability friction points, identified in discussion with a user who wanted to troubleshoot memory bloat:

• [x] Taking snapshots automatically should be controllable with settings
• [x] Persist memory screen settings.
• [ ] The usability of control pane: go/devtools-memory-coltrolpane-ux (discussion triggered by the request to show/hide chart pane)

Discussions:

• go/dart-devtools-memory-tooling-improvements
• go/devtools-memory-page-vision