Add Customizable Window Headers aconfig flag

Used to allow apps to customize the appearance of the caption bar
(opaque/transparent, light/dark foreground), as well as to introduce
inset bounding rectangles, which an app can use to understand which
areas of an inset are "empty" and thus could have app content placed
inside it.

Bug: b/316387515
Test: m
Change-Id: I239cfb151b4829da1fe7d4a954989c3179130fe6