fix(checkbox, radio): Fix checkbox label not read in JAWS

[tinayuangao]

Fixes #3584 #3586

[jelbourn]

Can you describe the problem this is fixing? According to the HTML spec, adding for should be equivalent to the form control being inside of the <label>

[tinayuangao]

The problem is JAWS doesn't read the label for radio button or checkbox. Moving input element out of label solves this problem. But according to spec, we could use label in both ways.

[jelbourn]

Can you reproduce the behavior on a simple html page with

<label>
  Active
  <input type="checkbox">
</label>

Or does it only happen w/ the material components?

[tinayuangao]

It only happen w/ the material components.

<md-checkbox>Active</md-checkbox> doesn't work while
<md-checkbox>Active<span>more info</span></md-checkbox> works.

[jelbourn]

LGTM

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