Fix empty keyguard for once and all

When a Keyguard exit animation was played while the screen was
turning off, the window animation was cancelled. However, since
we didn't reset mPostKeyguardExitAnimation, we thought we need to
apply it again, with a start time of Long.MIN_VALUE. For some reason,
this animation never ended, and it was stuck at the beginning. Thus,
the alpha was 0 and the user just saw a blank wallpaper.

In addition, we also forbid starting the Keyguard exit animation when
the screen is about to turn off.

Bug: 21124013
Change-Id: Ief022c5bd758a9fedcaab3fe9fc385fa4234b37f