This change documents that some system services are not available
to instant apps due to the restricted security sandbox they run in.
Also ensure that if target SDK is P WallpaperManager is null if the
backing system service cannot be looked up to be consistent with
the way other system services behave.
Test: Manual
Fixes: 70984822
Fixes: 69848394
Change-Id: I9aae884d5840c92136c7d713fac87e7732dc2642