[EuiAccordion] Children are still present in keyboard tab order when accordions are closed

[cee-chen]

Bug originally found in #6904 (comment)

Repro steps:

1. Go to https://codesandbox.io/s/withered-moon-hc3ft6?file=/demo.js
2. Close the top-most parent accordion
3. Hit tab - you would expect tab to go to the next focusable item outside the accordion, but it instead goes to the 2 nested accordions inside the parent accordion

This issue does not appear to occur once the child initialIsOpen accordions are collapsed, so something appears to be going wrong with that logic within EuiAccordion.

The above statement was incorrect, this issue also occurs for nested accordions that default to close.