demos(select): select header example is not accessible

[Splaktar]

Bug, feature request, or proposal:

As mentioned in #10342 (comment), the current md-select-header demo is not accessible and will lead developers to implement inaccessible select search boxes in their apps.

What is the expected behavior?

All of the demos should be using accessibility best practices and be considered 'accessible'.

What is the current behavior?

Currently the demo just sticks an input in the md-select-header. While that input has a placeholder, it lacks a number of the accessibility features of md-input. There may also be some accessibility issues in the general interaction with the component and initial focus state as described in #9146.

CodePen and steps to reproduce the issue:

CodePen Demo which shows your issue:

https://material.angularjs.org/latest/demo/select#select-header

Detailed Reproduction Steps:

Test with screen reader.

What is the use-case or motivation for changing an existing behavior?

To align with the accessibility goals of this project.

Which versions of AngularJS, Material, OS, and browsers are affected?

AngularJS 1.1.7

Is there anything else we should know? Stack Traces, Screenshots, etc.

No