no setters for @JavaOnlyImmutable union

AIDL compiler doesn't generate setters for a union-generated class when
a union is annotated @JavaOnlyImmutable.

Bug: 171637180
Test: aidl_unittests & aidl_integration_test
Change-Id: Id8290cf86f2ba5e593c930727a3741273e615ff0
diff --git a/aidl_unittest.cpp b/aidl_unittest.cpp
index d55460f..3635ad2 100644
--- a/aidl_unittest.cpp
+++ b/aidl_unittest.cpp
@@ -3189,16 +3189,18 @@
   private Object _value;
 
   public Foo() {
-    int[] value = {42};
-    _set(ns, value);
+    int[] _value = {42};
+    this._tag = ns;
+    this._value = _value;
   }
 
   private Foo(android.os.Parcel _aidl_parcel) {
     readFromParcel(_aidl_parcel);
   }
 
-  private Foo(int tag, Object value) {
-    _set(tag, value);
+  private Foo(int _tag, Object _value) {
+    this._tag = _tag;
+    this._value = _value;
   }
 
   public int getTag() {
@@ -3344,9 +3346,9 @@
     throw new IllegalStateException("unknown field: " + _tag);
   }
 
-  private void _set(int tag, Object value) {
-    this._tag = tag;
-    this._value = value;
+  private void _set(int _tag, Object _value) {
+    this._tag = _tag;
+    this._value = _value;
   }
 }
 )";