Don't apply animations to windows that are showing

If a window is showing and not obscured by the keyguard, for
example windows on other displays, don't apply the enter animation.
This prevents jank on secondary displays when the keyguard is
unlocked.

Fixes bug 18689396

Change-Id: I2deb3030e6e887f57af9a5f1d42b1c71f06f7eeb