Support selectable={true} property on Text fields on Android.

Summary:
Explain the **motivation** for making this change. What existing problem does the pull request solve?

This adds support for a text field that the user can click-and-drag to select text (which can then be copied using the native selected-text-hover-widget).

I'd love to add this to iOS too, but iOS appears to draw glyphs directly to the screen for its <Text> widget (versus using UITextField), so it might be too difficult to support there. But at least I can support my Android users with this change.

Let me know if/what kind of "demonstrate the code is solid" you would like for this. A screenshot of selected text with this property set?
Closes https://github.com/facebook/react-native/pull/8028

Differential Revision: D3474196

Pulled By: bestander

fbshipit-source-id: 8d3656681265a0e6229bfa13ff2ae021e894d3cd

ReactAndroid/src/main/java/com/facebook/react/views/text/ReactTextViewManager.java

@@ -112,6 +112,11 @@ public class ReactTextViewManager extends BaseViewManager<ReactTextView, ReactTe
     }
   }
 
+  @ReactProp(name = "selectable")
+  public void setSelectable(ReactTextView view, boolean isSelectable) {
+    view.setTextIsSelectable(isSelectable);
+  }
+
   @Override
   public void updateExtraData(ReactTextView view, Object extraData) {
     ReactTextUpdate update = (ReactTextUpdate) extraData;