Remove asset_id and url from OnlineWallpaperParams.

This starts the work for b/279781227, where the hub (or any caller
for that matter) can just pass the unit_id of the selected online
wallpaper to WallpaperController along with all possible variants.
WallpaperController then picks among the variants itself internally
before putting all of this data into a WallpaperInfo. Passing the
asset_id and url was redundant and forced the caller to pick the
variant itself, spreading this responsibility between both the
caller and WallpaperController.

Bug: b:279781227
Change-Id: I14469dec7c5d29c74620379ef8415531bc3fd562
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4980002
Commit-Queue: Eric Sum <esum@google.com>
Reviewed-by: Jason Thai <jasontt@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1219209}