Generate a CREATOR for each Parcelable message.

This is less ideal from a dex count perspective because it requires a
new variable for each message, and because most apps have proguard
rules that will ensure that CREATOR classes are retained.

However, it is required to be able to use nano protos inside of AIDL
files, as the autogenerated AIDL code fails to compile otherwise. This
is a substantial benefit as it allows for backwards-compatible
parameters and return types in AIDL methods along the lines of
safeparcel.

Bug: 19084705
Change-Id: I66a2c0424b96cf8ff6b631b186cc4f9407dfc1f4

src/google/protobuf/compiler/javanano/javanano_message.cc

@@ -156,6 +156,17 @@ void MessageGenerator::Generate(io::Printer* printer) {
   }
   printer->Indent();
 
+  if (params_.parcelable_messages()) {
+    printer->Print(
+      "\n"
+      "// Used by Parcelable\n"
+      "@SuppressWarnings({\"unused\"})\n"
+      "public static final android.os.Parcelable.Creator<$classname$> CREATOR =\n"
+      "    new com.google.protobuf.nano.android.ParcelableMessageNanoCreator<\n"
+      "        $classname$>($classname$.class);\n",
+      "classname", descriptor_->name());
+  }
+
   // Nested types and extensions
   for (int i = 0; i < descriptor_->extension_count(); i++) {
     ExtensionGenerator(descriptor_->extension(i), params_).Generate(printer);