Making KeyguardView slippery fixes 5501105

Setting the slippery flag keeps this window from processing
events occurring in places where another window is on top of it.
Specifically, if someone clicks above the pattern/FaceUnlock portion
of the window and then drags down onto FUL, this blocks the
underlying lockscreen from processing those touch events.

Change-Id: Ia7038841df0044f66220af8a00c7845b2293c2a4