Introduce integer constants

We can now declare a "const int" variable in an AIDL interface, in
addition to methods. These constants will become static members of the
interface/base class in the generated code.

Change-Id: I6b690ecbbe2acae37abb106510e42283f0753f26
Test: Unit and integration tests updated and pass
Bug: 23600061
Signed-off-by: Casey Dahlin <sadmac@google.com>
diff --git a/ast_java.h b/ast_java.h
index 267faba..f852e88 100644
--- a/ast_java.h
+++ b/ast_java.h
@@ -356,6 +356,17 @@
     void Write(CodeWriter* to) const override;
 };
 
+struct Constant : public ClassElement
+{
+    string name;
+    int value;
+
+    Constant() = default;
+    virtual ~Constant() = default;
+
+    void Write(CodeWriter* to) const override;
+};
+
 struct Class : public ClassElement
 {
     enum {