Add TextInput controlled selection prop on Android

Summary:
Android PR for TextInput selection, based on the iOS implementation in #8958.

** Test plan **
Tested using the text selection example in UIExplorer.
Closes https://github.com/facebook/react-native/pull/8962

Differential Revision: D3819285

Pulled By: andreicoman11

fbshipit-source-id: 9a2408af2a8b694258c88ab5c46322830c71452a

Libraries/Components/TextInput/TextInput.js

@@ -393,7 +393,6 @@ const TextInput = React.createClass({
     /**
      * The start and end of the text input's selection. Set start and end to
      * the same value to position the cursor.
-     * @platform ios
      */
     selection: PropTypes.shape({
       start: PropTypes.number.isRequired,
@@ -679,6 +678,10 @@ const TextInput = React.createClass({
       children = <Text>{children}</Text>;
     }
 
+    if (props.selection && props.selection.end == null) {
+      props.selection = {start: props.selection.start, end: props.selection.start};
+    }
+
     const textContainer =
       <AndroidTextInput
         ref={this._setNativeRef}