Implement unlock hint.

Before, users didn't have any clue how to unlock their phone. Now
they have. Also, a tap now dismisses the notification shade in the
unlocked state.

Bug: 14080971
Bug: 15189435
Bug: 15282191
Bug: 15126962

Change-Id: I779344b043b4415809dd98217b7cb9ff6d57fa3e