Multiple bug fixes for resizable container

[chandlerprall]

Fixes #3698

Summary

• Fixed the bug reported in EuiResizableContainer having nested instances (vertical and horizontal) doesn't work #3698 where a nested panel would not resize. The issue came from looking at all resizable buttons in a container instead of only the ones directly used by a container
• Fixed a bug where resizing by arrow keys did nothing in an un-controlled usage
• Fixed a bug which impacted the last docs' example where resizable buttons of different sizes were improperly assumed to be of the same size

Replaced resizable_container_basic with the following for testing:

import React from 'react';
import { EuiText, EuiResizableContainer } from '../../../../src/components';
import { fake } from 'faker';

const text = (
  <>
    <p>{fake('{{lorem.paragraphs}}')}</p>
    <p>{fake('{{lorem.paragraphs}}')}</p>
    <p>{fake('{{lorem.paragraphs}}')}</p>
  </>
);

export default () => (
  <div>
    <EuiResizableContainer style={{ height: '600px' }}>
      {(EuiResizablePanel, EuiResizableButton) => (
        <>
          <EuiResizablePanel initialSize={60} minSize="20%">

            <EuiResizableContainer direction="vertical" style={{ height: '600px' }}>
              {(EuiResizablePanel, EuiResizableButton) => (
                <>
                  <EuiResizablePanel initialSize={50} minSize="20%">
                    <EuiText>
                      <p>Top Panel</p>
                    </EuiText>
                  </EuiResizablePanel>

                  <EuiResizableButton />

                  <EuiResizablePanel initialSize={50} minSize="10%">
                    <EuiText>
                      <p>Bottom Panel</p>
                    </EuiText>
                  </EuiResizablePanel>
                </>
              )}
            </EuiResizableContainer>

          </EuiResizablePanel>

          <EuiResizableButton />

          <EuiResizablePanel initialSize={40} minSize="10%">
            <EuiText>
              <p>{text}</p>
            </EuiText>
          </EuiResizablePanel>
        </>
      )}
    </EuiResizableContainer>
  </div>
);

Checklist

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

• Checked in Safari and Firefox
  - [ ] Props have proper autodocs
  - [ ] Added documentation
  - [ ] Checked Code Sandbox works for the any docs 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

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3699/

[thompsongl]

🙌
Tested locally with the provided example.
Tested keyboard nav across the board.