Make SystemService constructor take a Context.

This change simplifies the process of initializing a SystemService
by folding the onCreate() step back into the constructor.  It removes
some ambuiguity about what work should happen in the constructor and
should make it possible for services to retain most of their final
fields after refactoring into the new pattern.

Change-Id: I25f41af0321bc01898658ab44b369f9c5d16800b