Prevent EuiDataGrid from auto-focusing

[chandlerprall]

Summary

Fixes #2862

Refactored the focusCell stuff a bit to initialize the data grid with no focus, which prevents autofocusing on the first header cell. This is testable on the Data grid and (especially) Data grid control columns examples.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile

• Checked in IE11 and Firefox
  - [ ] Props have proper autodocs
  - [ ] Added documentation examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[myasonik]

Tested locally and everything works as expected! 🎉

[snide]

Tested locally for functionality. Works as described. Tested other scenarios like pagination and opening the menus to make sure focus acted as expected

[thompsongl]

👍 useFocus

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2872/