Phase 1 of refactoring SystemServer.

SystemServer is currently a monolithic class that brings up key system
services. This change is the first phase of refactoring it to be more
configurable. Specifically, it adds a set of on/off switches used to control
startup of individual services. Future plans include finer grained controls
and a more explicit and consistent startup sequence for these services.

Change-Id: I7299f5ce7d7b74a34eb56dffb788366fbc058532