Preparing to finalize @SystemApi

- MODULE_APPS is considered to be too confusing, we're going to remove it, and
may revive it once ART is able to support runtime protection

- Change SYSTEM_SERVER to a "client".

Test: Build / treehugger
Bug: 148177503
Change-Id: Id847e29ab211c4c009c234a2e1e469575914e12e