Document which sys services can be accessed by instant apps

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