Fix blank Keyguard #4

When we were fading out the preview for phone/camera to crossfade to
the real application, it might have been that showKeyguard was
called in the middle of this transition. This change makes sure that
we cancel this animation when showing the keyguard.

Bug: 17439581
Change-Id: I89cacf7ecf43d37ea979418b976390272510a09d