Fix WallpaperManager javadoc for new U methods

The message about permissions for old methods was wrongly copied for the
new U methods. Don't mention versions <= S and T for methods that have
been added in U.

Also add a couple missing "{".

Bug: 285731765
Test: none, only javadoc changes
Change-Id: I54085b28562a286584208d7dec0aee88d1cb43fa