The OS will not distinguish between the physical underlying wallpaper
image and the target displayable subrect of that image. This will enable
more pleasing conveyance of backed-up wallpaper imagery between dissimilar
devices. The allow-backup hint is a way for wallpaper sources to ensure
that the OS doesn't back up imagery that should not be propagated across
devices.
(NB: the backing implementation isn't in place yet; this is just to
permit forward-looking work on client apps in the interim.)
Bug 25454501
Bug 25453848
Change-Id: Id014d552ae509f659992d57b915ef95c5d4b8d1a
ffa6a88658