init: fix restarting of subcontext

When the subcontext code was redone to allow only one subcontext
(vendor_init), the code for restarting it and for terminating it
during shutdown was not updated, resulting in it not working.

Bug: 155203339
Test: kill subcontext init and notice it restart
Test: subcontext init stops during shutdown
Change-Id: Ib77f59d1e7be0ffcfd3f31c8450dc022c20bb322
diff --git a/init/subcontext.h b/init/subcontext.h
index 5e1d8a8..788d3be 100644
--- a/init/subcontext.h
+++ b/init/subcontext.h
@@ -60,7 +60,8 @@
 };
 
 int SubcontextMain(int argc, char** argv, const BuiltinFunctionMap* function_map);
-std::unique_ptr<Subcontext> InitializeSubcontext();
+void InitializeSubcontext();
+Subcontext* GetSubcontext();
 bool SubcontextChildReap(pid_t pid);
 void SubcontextTerminate();