Cleanup Keyguard handling when turning off screen

Before, Keyguard was shown whenever PhoneWindowManager went to sleep.
However, this was too early on some devices, and too late on others.
Now, the callbacks are split up into two phases: startedGoingToSleep
and finishedGoingToSleep. In the first callback, we decide what to
do and play the lock sound if necessary. In the second callback, we
actually show the Keyguard, or reset its state necessary, so it
doesn't interfere with the screen off animation.

Bug: 17929748
Bug: 20782303
Change-Id: I4c4406595b07b7589d64f380cd3fa96bed3d20ee