Add logs to WallpaperManagerService when setting wallpaper

Bug: 151817908
Test: build

Most logs in WallpaperManagerService are under 'DEBUG' flag. This makes
it harder to debug failures to set wallpaper after it has been restored
from a backup. Since setting a wallpaper is an infrequent operation, add
some more logs.

Change-Id: I3cfbec63ccfe64697a177fcb73edd424fb94cb8b