init: Cleanly shut down subcontext processes

This change adds an explicit cleanup for the subcontext processes and
avoids them from respawning, which causes a bunch of LOG(FATAL)s when
the system is going down.

Bug: 80425914
Test: kill -TERM $INIT_PID, no crashes for subcontext inits

Change-Id: I135191d959c1dd921b102af316b24d2bc161d6c9
diff --git a/init/subcontext.h b/init/subcontext.h
index 22d7d43..628fd50 100644
--- a/init/subcontext.h
+++ b/init/subcontext.h
@@ -63,6 +63,7 @@
 int SubcontextMain(int argc, char** argv, const KeywordFunctionMap* function_map);
 std::vector<Subcontext>* InitializeSubcontexts();
 bool SubcontextChildReap(pid_t pid);
+void SubcontextTerminate();
 
 }  // namespace init
 }  // namespace android