Treat R8 warnings as failures for system_server

Treating warnings as failures avoids unexpected issues at build and
runtime, particularly for missing class references. Unblock this by
adding explicit top-level references to several stub classes, and
suppressing a warning for optional jacococ class references (guarded
by a runtime check in CoverageService).

Test: m services (also w/ EMMA_INSTRUMENT(_FRAMEWORK)=true)
Bug: 242088131
Change-Id: Iee02a30eeccdcf785e90438d1557867ffda659e7