bug(datepicker): previous/next month buttons missing hover state

[mmalerba]

Repro: See basic datepicker example at https://material.angular.io/components/datepicker/overview

[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.}