Screen magnifier should handle window rebuilds correctly.

1. The way for computing the magnified region was simplistic and
   incorrect. It was ignoring window layering resulting in broken
   behavior. For example, if the IME is up, then the everything else
   is magnifed and the IME not. Now the keyguard appears and covers
   the IME but the magnified region does not expand while it would
   since the keyguard completely covers the not magnified IME window.

bug:7138937

Change-Id: I21414635aefab700ce75d40f3e913c1472cba202