feat(autocomplete): add support for aria-describedby

[Splaktar]

PR Checklist

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• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Applying aria-describedby to an md-autocomplete would not cause the attribute to be forwarded down to the native input element. Thus it would have no effect.

Issue Number:
Fixes #11004

What is the new behavior?

Applying aria-describedby to an md-autocomplete causes the attribute to be forwarded down to the native input element. This causes the contents of all elements with IDs in the list to be read after the autocomplete is selected and the label and state are announced.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[mmalerba]

I'm seeing some accessibility test failures during presubmit:

Error: AX_ARIA_04 (ARIA state and property values must be valid) failed on the following element: #input-1

Error: AX_ARIA_02 (ARIA attributes which refer to other elements by ID should refer to elements which exist in the DOM) failed on the following element: #input-1

[Splaktar]

@mmalerba OK, that was a good catch. We don't want to set aria-describedby="false" when the user didn't define the attribute. This is now fixed. Please re-submit.

[mmalerba]

@Splaktar please rebase

[Splaktar]

@mmalerba rebased.