Store decor rects per window for transition cropping.

Instead of keeping a single global system decor rect around
in WindowManagerService, calculate and store policy-defined
system-decor frame for each window.

The per-window decor rect is useful for smooth transitions, since it
determines window cropping during transition animations.

Bug:10938001
Change-Id: Ice6652aa5946027c45c0b7ab4e46473a0f8e3f90