Codegen: Add JNI C++ generator for TurboModule subclasses

Summary:
This adds the C++ classes for Android, just like checked in here: https://github.com/facebook/react-native/tree/master/ReactAndroid/src/main/java/com/facebook/fbreact/specs/jni

The whitespace/formatting is slightly different but the functionality should be preserved.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D23489368

fbshipit-source-id: 60c112a48b6de2d2eb19f7d1bcc894048334610f

scripts/generate-native-modules-specs-cli.js

@@ -38,7 +38,7 @@ function generateSpec(platform, schemaPath, outputDirectory) {
   RNCodegen.generate(
     {libraryName, schema, outputDirectory: tempOutputDirectory, moduleSpecName},
     {
-      generators: ['modules'],
+      generators: platform === 'android' ? ['modulesAndroid'] : ['modules'],
     },
   );
 
@@ -73,6 +73,22 @@ function generateSpec(platform, schemaPath, outputDirectory) {
           `${outputDirectory}/${f}`,
         );
       });
+
+    // And all C++ files for JNI.
+    const jniOutputDirectory = `${outputDirectory}/jni`;
+    mkdirp.sync(jniOutputDirectory);
+    files
+      .filter(
+        f =>
+          f.startsWith(moduleSpecName) &&
+          (f.endsWith('.h') || f.endsWith('.cpp')),
+      )
+      .forEach(f => {
+        fs.copyFileSync(
+          `${tempOutputDirectory}/${f}`,
+          `${jniOutputDirectory}/${f}`,
+        );
+      });
   }
 }