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We set the system_server classpath in the environment
(like we do with BOOTCLASSPATH). After the zygote forks
the system_server, we dexopt the classpath (if needed)
and then launch the system server with the correct
PathClassLoader. This needed several small / medium
refactorings :

- The logic for connecting to installd is now in a separate
  class and belongs in the system_server.
- SystemService / SystemServiceManager have now moved to
  classes.jar. They are only used from there, and since they
  use Class.forName, we want them to be loaded by the
  system_server classloader, and not the bootclassloader.
- BootReceiver now moves to frameworks.jar, because it is
  used by ActivityThread and friends.

bug: 16555230

Change-Id: Ic84f0b2baf611eeedff6d123cb7191bb0259e600