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/builtins.h b/init/builtins.h
index 7bbf6aa..f0ff1eb 100644
--- a/init/builtins.h
+++ b/init/builtins.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef _INIT_BUILTINS_H
-#define _INIT_BUILTINS_H
+#pragma once
 
 #include <functional>
 #include <map>
@@ -31,18 +30,16 @@
 
 using BuiltinFunction = std::function<Result<void>(const BuiltinArguments&)>;
 
-using KeywordFunctionMap = KeywordMap<std::pair<bool, BuiltinFunction>>;
-class BuiltinFunctionMap : public KeywordFunctionMap {
-  public:
-    BuiltinFunctionMap() {}
-
-  private:
-    const Map& map() const override;
+struct BuiltinFunctionMapValue {
+    bool run_in_subcontext;
+    BuiltinFunction function;
 };
 
+using BuiltinFunctionMap = KeywordMap<BuiltinFunctionMapValue>;
+
+const BuiltinFunctionMap& GetBuiltinFunctionMap();
+
 extern std::vector<std::string> late_import_paths;
 
 }  // namespace init
 }  // namespace android
-
-#endif