Open souce the Android Dialog module

Summary:
public

The `DialogModule` requires `android.support.v4.app.FragmentManager` which means
every app that wants to use Dialogs would need to have its Activity extend the legacy
`android.support.v4.app.FragmentActivity`.

This diff makes the `DialogModule` work with both the Support `FragmentManager`
(for AdsManager & potentially other fb apps) and the `android.app.FragmentManager`
(for new apps with no legacy dependencies).

Also wrap the native module in the same `Alert` API that we have on iOS and provide
a cross-platform example. In my opinion the iOS Alert API is quite nice and easy to use.

We still keep `AlertIOS` around because of its `prompt` function which is iOS-specific
and also for backwards compatibility.

Reviewed By: foghina

Differential Revision: D2647000

fb-gh-sync-id: e2280451890bff58bd9c933ab53cd99055403858

ReactAndroid/src/main/java/com/facebook/react/shell/MainReactPackage.java

@@ -17,6 +17,7 @@ import com.facebook.react.ReactPackage;
 import com.facebook.react.bridge.JavaScriptModule;
 import com.facebook.react.bridge.NativeModule;
 import com.facebook.react.bridge.ReactApplicationContext;
+import com.facebook.react.modules.dialog.DialogModule;
 import com.facebook.react.modules.fresco.FrescoModule;
 import com.facebook.react.modules.intent.IntentModule;
 import com.facebook.react.modules.location.LocationModule;
@@ -53,6 +54,7 @@ public class MainReactPackage implements ReactPackage {
     return Arrays.<NativeModule>asList(
       new AsyncStorageModule(reactContext),
       new ClipboardModule(reactContext),
+      new DialogModule(reactContext),
       new FrescoModule(reactContext),
       new IntentModule(reactContext),
       new LocationModule(reactContext),