[Pixel Diff Ash] Set wallpaper during pixel test setup correctly

The wallpaper type should be `kOneShot` instead of `kDefault`. This
change guarantees that the custom wallpaper set by the pixel test
helper is persistent after changing display metrics such as altering
the display size or adding an additional display.

Given that currently the one shot wallpaper cannot be blurred or
shielded, this CL adds an API to allow the exception in pixel tests.

This CL should not bring any noticeable changes.

Bug: None
Change-Id: I9212bab8ae1549ec52795dfbbae69c2bbd126989
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4023978


Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Jeffrey Young <cowmoo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1072418}