Fix wallpaper restore from pre-N devices

The previous path "worked" but left the wallpaper bookkeeping with
incorrect SELinux labeling, which led to cascade failures later on
when apps tried to set new system wallpaper imagery or read the
current image for the picker UI.

We now (a) explicitly label wallpaper files in all change cases,
(b) let the restored imagery flow through the full crop path, as
should have been done in the first place, and as a result (c) lift
the size restrictions on the source image, because now we are doing
a device-appropriate scaling operation on the image.

The "when to crop/scale" test has been slightly relaxed such that
a restored image of exactly the right size now longer triggers a
superfluous factor-1.0 scaling operation.

Bug 30361282

Change-Id: I9a305eea2227952493f64ab78862648cafb816ff