fix(select): support use inside a custom value accessor

[kara]

Fixes #2609.

[jelbourn]

LGTM

Do other components need to do anything like this as well?

[kara]

@jelbourn I believe this is the only component right now that sets the value accessor directly. mdInput uses the default value accessor for text fields and the others use the provider syntax (which will work correctly).

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}