init: simplify keyword_map

I've heard that keyword_map is too complex, in particular the tuple
and the pair in BuiltinFunctionMap, so this change removes a lot of
that complexity and, more importantly, better documents how all of
this works.

Test: boot, init unit tests

Change-Id: I74e5f9de7f2ec524cb6127bb9da2956b5f307f56
diff --git a/init/main.cpp b/init/main.cpp
index 2ce46ef..38bc74b 100644
--- a/init/main.cpp
+++ b/init/main.cpp
@@ -60,7 +60,7 @@
     if (argc > 1) {
         if (!strcmp(argv[1], "subcontext")) {
             android::base::InitLogging(argv, &android::base::KernelLogger);
-            const BuiltinFunctionMap function_map;
+            const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();
 
             return SubcontextMain(argc, argv, &function_map);
         }