[Zoom-out 1/N] Add API in WallpaperManager

Add an API in WallpaperManager (client and server) and
WallpaperService to pass a wallpaper zoom value to the
wallpaper service and engine.

Bug: 146387434
Test: (added testDeliversZoomChanged)
Test: atest WallpaperServiceTest
Change-Id: Idd9ea2aefb845ad1d330cbdd6e088b926bcfece7