Export the DevSettings module, add addMenuItem method (#25848)

Summary:
I wanted to configure the RN dev menu without having to write native code. This is pretty useful in a greenfield app since it avoids having to write a custom native module for both platforms (and might enable the feature for expo too).

This ended up a bit more involved than planned since callbacks can only be called once. I needed to convert the `DevSettings` module to a `NativeEventEmitter` and use events when buttons are clicked. This means creating a JS wrapper for it. Currently it does not export all methods, they can be added in follow ups as needed.

## Changelog

[General] [Added] - Export the DevSettings module, add `addMenuItem` method
Pull Request resolved: https://github.com/facebook/react-native/pull/25848

Test Plan:
Tested in an app using the following code.

```js
if (__DEV__) {
 DevSettings.addMenuItem('Show Dev Screen', () => {
    dispatchNavigationAction(
      NavigationActions.navigate({
        routeName: 'dev',
      }),
    );
  });
}
```

Added an example in RN tester

![devmenu](https://user-images.githubusercontent.com/2677334/62000297-71624680-b0a1-11e9-8403-bc95c4747f0c.gif)

Differential Revision: D17394916

Pulled By: cpojer

fbshipit-source-id: f9d2c548b09821c594189d1436a27b97cf5a5737

ReactAndroid/src/main/java/com/facebook/react/CoreModulesPackage.java

@@ -139,7 +139,7 @@ import java.util.Map;
       case DeviceEventManagerModule.NAME:
         return new DeviceEventManagerModule(reactContext, mHardwareBackBtnHandler);
       case DevSettingsModule.NAME:
-        return new DevSettingsModule(mReactInstanceManager.getDevSupportManager());
+        return new DevSettingsModule(reactContext, mReactInstanceManager.getDevSupportManager());
       case ExceptionsManagerModule.NAME:
         return new ExceptionsManagerModule(mReactInstanceManager.getDevSupportManager());
       case HeadlessJsTaskSupportModule.NAME:

ReactAndroid/src/main/java/com/facebook/react/devsupport/DevInternalSettings.java

@@ -123,6 +123,11 @@ public class DevInternalSettings
     return mPreferences.getBoolean(PREFS_START_SAMPLING_PROFILER_ON_INIT, false);
   }
 
+  @Override
+  public void addMenuItem(String title) {
+    // Not supported.
+  }
+
   public interface Listener {
     void onInternalSettingsChanged();
   }

ReactAndroid/src/main/java/com/facebook/react/modules/debug/DevSettingsModule.java

@@ -6,23 +6,31 @@
  */
 package com.facebook.react.modules.debug;
 
-import com.facebook.react.bridge.BaseJavaModule;
+import com.facebook.react.bridge.Arguments;
+import com.facebook.react.bridge.ReactApplicationContext;
+import com.facebook.react.bridge.ReactContextBaseJavaModule;
 import com.facebook.react.bridge.ReactMethod;
 import com.facebook.react.bridge.UiThreadUtil;
+import com.facebook.react.bridge.WritableMap;
+import com.facebook.react.devsupport.interfaces.DevOptionHandler;
 import com.facebook.react.devsupport.interfaces.DevSupportManager;
 import com.facebook.react.module.annotations.ReactModule;
+import com.facebook.react.modules.core.DeviceEventManagerModule.RCTDeviceEventEmitter;
 
 /**
  * Module that exposes the URL to the source code map (used for exception stack trace parsing) to JS
  */
 @ReactModule(name = DevSettingsModule.NAME)
-public class DevSettingsModule extends BaseJavaModule {
+public class DevSettingsModule extends ReactContextBaseJavaModule {
 
   public static final String NAME = "DevSettings";
 
   private final DevSupportManager mDevSupportManager;
 
-  public DevSettingsModule(DevSupportManager devSupportManager) {
+  public DevSettingsModule(
+      ReactApplicationContext reactContext, DevSupportManager devSupportManager) {
+    super(reactContext);
+
     mDevSupportManager = devSupportManager;
   }
 
@@ -63,4 +71,20 @@ public class DevSettingsModule extends BaseJavaModule {
   public void toggleElementInspector() {
     mDevSupportManager.toggleElementInspector();
   }
+
+  @ReactMethod
+  public void addMenuItem(final String title) {
+    mDevSupportManager.addCustomDevOption(
+        title,
+        new DevOptionHandler() {
+          @Override
+          public void onOptionSelected() {
+            WritableMap data = Arguments.createMap();
+            data.putString("title", title);
+            getReactApplicationContext()
+                .getJSModule(RCTDeviceEventEmitter.class)
+                .emit("didPressMenuItem", data);
+          }
+        });
+  }
 }

ReactAndroid/src/main/java/com/facebook/react/modules/debug/interfaces/DeveloperSettings.java

@@ -35,4 +35,7 @@ public interface DeveloperSettings {
 
   /** @return Whether Start Sampling Profiler on App Start is enabled. */
   boolean isStartSamplingProfilerOnInit();
+
+  /** Add an item to the dev menu. */
+  void addMenuItem(String title);
 }