Allow wallpaper surface in preview to scale to a rect

Add a method to scale and translate a wallpaper's surface to
a given Rect (relative to the Window) when in preview mode.
This can be used by the WallpaperPicker to provide a scaled
preview of a given wallpaper.

Bug: 151868130
Test: Manually verify that WallpaperPicker preview looks correct
Change-Id: I7bfbe0690bf9c93738968bd1f630489a082ff23b