added accessibility props for touchables

Summary:
accessibilityLabels are missing in these touchable*.js files.
for #5322
ide This is not tested yet. I will update with test.
Closes https://github.com/facebook/react-native/pull/5346

Reviewed By: svcscm

Differential Revision: D2882061

Pulled By: gkassabli

fb-gh-sync-id: dff0ef373e5f5895027cb1cc08c8887a6ace8eee

Examples/UIExplorer/TouchableExample.js

@@ -144,6 +144,9 @@ var TouchableFeedbackEvents = React.createClass({
           <TouchableOpacity
             style={styles.wrapper}
             testID="touchable_feedback_events_button"
+            accessibilityLabel="touchable feedback events"
+            accessibilityTraits="button"
+            accessibilityComponentType="button"
             onPress={() => this._appendEvent('press')}
             onPressIn={() => this._appendEvent('pressIn')}
             onPressOut={() => this._appendEvent('pressOut')}