Add methods to coordinate unlock animation.

Introduce IWindowManager.keyguardGoingAway to notify that Keyguard
wants to dismiss it self. This method starts the state machine in
WindowAnimator which animates in the activity behind the keyguard.
Animating out the keyguard is done by the StatusBar/Keyguard
software when it receives the startKeyguardExitAnimation() callback.

Bug: 14118756

Change-Id: Id3b8f41189410bad808b4892fbec74245e59efce