Add API comment for new API in WallpaperService.

Supplement some comment to let developer know how to use new API
WallpaperInfo#supportsMultipleDisplays.

Also add a nullability annotation and comment for
WallpaperService.Engine#getDisplayContext.

Fix: 124302009
Fix: 123661589
Test: Build pass and read comment.

Change-Id: Id6040eff12495838f21f539b7c68d6743c9baf74