[ML] New Job should not auto-focus Saves Searches Filter without giving proper screen reader context

[aphelionz]

Steps to reproduce (assumes ChromeVox or similar)

1. Open Kibana in a new tab
2. Tab to Machine Learning
3. Tab to "+ Create New Job" button
4. Press enter

Actual Result
5. The Saved searches filter is auto-focused and read only:

Saved Searches Filter ellipses, edit text

Expected Result
Either the element should not be auto focused or the screen reader label should be edited to provide more context as to where the user is and what they are doing, perhaps:

Start typing to select a saved search to use for your new machine learning job

Category: #18866 Kibana 6.3 Meta Issues
Relevant WCAG Criteria: 3.2.2 Predictable on Input

[elasticmachine]

Pinging @elastic/ml-ui