Correct getWallpaperInfo javadoc

We forgot to mention that before android 14, the permission
QUERY_ALL_PACKAGES is required in order to use getWallpaperInfo.

Also mention that we return null if the caller doesn't have the
permission.

Bug: 285731765
Test: NA; javadoc change
Change-Id: Ibad0701412838965f387978cda4703e598ea4fa4