Occasionally triple tap on the keyboard toggles screen magnification.

1. Sometimes unlocking the device when the IME is up and triple tapping on the keyboard
   toggles screen magnification. The core reason is that when the kayguard window is
   shown we hide all other windows and when it is hidden we show these windows. We did
   not notify the screen magnifier for windows being shown and hidden. Also when the
   windows are shown we may reassign layers to put the IME or the wallpaper in the
   right Z order. The screen magnifier is now notified upon such layer reassignment
   since window layers are used when computing the magnified region.

bug:7351531

Change-Id: I0931f4ba6cfa565d8eb1e3c432268ba1818feea6