update(autoComplete): allow clear button even if directive is disabled

[achuinard]

I am attempting to use md-autocomplete and 'lock' in a users selection once an autocomplete is selected. I ng-disable the directive accordingly. However, I still want the user to be able to clear the autocomplete, even if it is disabled - I simply don't want them to be able to edit the existing text. This pull request will still allow the clear button to be shown even if it is disabled. If the developer doesn't want a clear button when the control is disabled, they can simply update their boolean expression inside of the md-clear-button attribute.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[achuinard]

I signed it!

[googlebot]

CLAs look good, thanks!