Don't allow non-const pointers to Type instances

This makes the namespace into a read only datastructure of types
with a few methods to load types during parsing.

Bug: 24303749
Test: compiles, unittests

Change-Id: I0873e520d771b348a1a8ec9515b6e7b9a8a29298
diff --git a/generate_java.h b/generate_java.h
index 5fd92ed..12d9f57 100644
--- a/generate_java.h
+++ b/generate_java.h
@@ -28,7 +28,7 @@
     using Type = android::aidl::Type;
 
     VariableFactory(const string& base); // base must be short
-    Variable* Get(Type* type);
+    Variable* Get(const Type* type);
     Variable* Get(int index);
 private:
     vector<Variable*> m_vars;