Explicitly reference system services where possible

System services bundled in services.jar should be explicitly referenced
wherever possible. This avoids ambiguity about where the service is
defined and what guarantees are made at runtime for its inclusion, and
also provides an explicit reference that R8 can use to keep associated
services.

This change is effectively a no-op, as we still keep all SystemService
derivations. However, it will make it possible in a future update to
relax this keep rule and depend on build-time system feature defines to
gatekeep service references.

A follow-up change will tighten the semantics for starting services via
the string-based class reflection lookup.

Bug: 203143243
Test: m + validate services.jar identical except for strings
Change-Id: If24d856df3da862d132671c912c1a4b68c8d46b5