[ML] (Accessibility) Machine Learning - Focus Order and focus visible

[barlowm]

Steps to reproduce
Open the "Single Metric Viewer" tab in Machine Learning.
Tab through the controls to the "Anomalies - Severity threshold" control
(Note: tabing through the controls, some controls have barely visible focus:

)

Press Enter to select the control to display possible values
Press ESC to close the control or ENTER to select a value
Next TAB should navigate to the "Interval" control, but focus returns to the "Home" control at the top of the page.

Kibana Version:
7.0

OS:
Windows

Browser:
Chrome

Screen reader: [if relevant]
N/A

Relevant WCAG Criteria: WCAG Quick Reference
2.4.3 Focus Order - Level A
2.4.7 Focus Visible - Leval AA

Relevant ARIA spec: wai-aria-practices
N/A

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

The loss of focus order on exiting the options in the severity control looks like it needs a fix to EuiSuperSelect - have raised elastic/eui#2708.