union: java backend

union is a parcelable which can hold only a single field with a tag.

Example:
  union Union {
    int n;
    String s;
  }

In Java, you can instantiate it with
- default constructor: init with first field
- value constructor: Union.n(42) or Union.s("abc")

You can query "tag" before getting the contents from it.
It also supports getter/setter.

Example:
  void foo(Union u) {
    if (u.getTag() == Union.n) {  // query
      int n = u.getN();           // getter
      ...
    }
    u.setS("abc");                // setter
  }

Bug: 150948558
Test: atest aidl_integration_test
Change-Id: I5c2d87e09462c0d3c6617d73fdf0e49c281d551e
diff --git a/generate_java.h b/generate_java.h
index d2c8a6d..3ed2546 100644
--- a/generate_java.h
+++ b/generate_java.h
@@ -40,6 +40,8 @@
 void generate_enum(const CodeWriterPtr& code_writer, const AidlEnumDeclaration* enum_decl,
                    const AidlTypenames& typenames);
 
+void generate_union(CodeWriter& out, const AidlUnionDecl* decl, const AidlTypenames& typenames);
+
 std::vector<std::string> generate_java_annotations(const AidlAnnotatable& a);
 
 }  // namespace java