Improve lock screen hide animation.

Fade the alpha quicker, to reduce the amount of dimming to
black we see in the middle.  Also start scaled a little
farther back; since in this case the lock screen is fully
opaque, we won't see the early part of the window animation
behind.

Change-Id: I65fcfbf2f27860abf6a1b225a44428d8a20cfb34