Fix getWallpaperInfo javadoc (again!)

A mistakes was made in ag/24905907: the mention "before Android U" was
accidentally added to the overload that was added in U.

Also fix a couple typos.

Bug: 285731765
Test: NA, javadoc change
Flag: NA
Change-Id: Ie927639822a247a19aa0e051b430bfe12b25d52b