Minor fixes for devices without WallpaperService.

- Fixed constructors so DisabledWallpaperManager doesn't throw exception
- Fixed SystemServiceRegistry.get(WallpaperManager.class) for instant apps

Bug: 138939803
Test: atest --instant CtsPermissionTestCases:android.permission.cts.ServicesInstantAppsCannotAccessTests#cannotGetWallpaperManager

Change-Id: I7cd6c65e9a693f3f1c4d5232420cc3f881d398c1