Add "required_parameters" to Annotation params

Some annotations must have a set of parameters. To model this
requirements, the schema for annotations has "required_parameters" now.

For example, @Backing for enum types should have "type" parameter even
though the annotation itself is optional.

AidlAnnotation::CheckValid() checks if there's no missing required
parameters.

Bug: none
Test: aidl_unittests
Change-Id: I50d82a1576f62bfdc2f220b66b7475cf74198e53
diff --git a/aidl_language.h b/aidl_language.h
index d8bb9d8..f19ef24 100644
--- a/aidl_language.h
+++ b/aidl_language.h
@@ -208,6 +208,8 @@
     std::map<std::string, std::string> supported_parameters;
 
     bool repeatable;
+
+    std::vector<std::string> required_parameters = {};
   };
   static const std::vector<Schema>& AllSchemas();