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

Examples/UIExplorer/UIExplorerList.android.js

@@ -38,6 +38,7 @@ var COMPONENTS = [
 
 var APIS = [
   require('./AccessibilityAndroidExample.android'),
+  require('./AlertExample').AlertExample,
   require('./BorderExample'),
   require('./ClipboardExample'),
   require('./GeolocationExample'),