Make <Modal> visible by default

Summary: Make <Modal> visible by default and fix the scenario where we present a modal immediately when adding it to the view hierarchy.

Closes #3724
Closes #2952

public

Reviewed By: nicklockwood

Differential Revision: D2595938

fb-gh-sync-id: 1571790d36fe486f1fbbed9f2d66f1e6add73d91

Examples/UIExplorer/ModalExample.js

@@ -143,6 +143,7 @@ var styles = StyleSheet.create({
   },
   innerContainer: {
     borderRadius: 10,
+    alignItems: 'center',
   },
   row: {
     alignItems: 'center',
@@ -158,6 +159,7 @@ var styles = StyleSheet.create({
     borderRadius: 5,
     flex: 1,
     height: 44,
+    alignSelf: 'stretch',
     justifyContent: 'center',
     overflow: 'hidden',
   },