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/ReactInstanceManager.java

@@ -186,7 +186,7 @@ public abstract class ReactInstanceManager {
     /**
      * Path to the JS bundle file to be loaded from the file system.
      *
-     * Example: {@code "assets://index.android.js" or "/sdcard/main.jsbundle}
+     * Example: {@code "assets://index.android.js" or "/sdcard/main.jsbundle"}
      */
     public Builder setJSBundleFile(String jsBundleFile) {
       mJSBundleFile = jsBundleFile;