Ravenwood test authors should only need to call setServicesRequired()
for the services they directly interact with, but often those
services will have indirect dependencies on other OS internals, and
those dependencies will evolve over time.

To handle this, we add the concept of dependencies to SystemService,
where a service declares the other services required to run.  (They
can choose to omit services from their dependencies where they
gracefully handle a missing service at runtime.)

Bug: 325506297
Test: atest RavenwoodServicesTest
Change-Id: I42187b3494fef499d619ad882891e832b0fd6bca