Allow for styling of ToolbarAndroid's overflow icon

Summary: Fixes #2858.

Here's a screenshot of the custom overflow icon in action:
<img width="638" alt="overflow" src="https://cloud.githubusercontent.com/assets/1309177/10567090/693e395a-75ae-11e5-84cd-20b19149c620.png">
Closes https://github.com/facebook/react-native/pull/3497

Reviewed By: svcscm

Differential Revision: D2559787

Pulled By: foghina

fb-gh-sync-id: f188711ec094af3fa307722527f22aefff722e64

Examples/UIExplorer/ToolbarAndroidExample.android.js

@@ -101,6 +101,12 @@ var ToolbarAndroidExample = React.createClass({
             title="Bunny and Hawk"
             style={styles.toolbar} />
         </UIExplorerBlock>
+        <UIExplorerBlock title="Toolbar with custom overflowIcon">
+          <ToolbarAndroid
+            actions={toolbarActions}
+            overflowIcon={require('./bunny.png')}
+            style={styles.toolbar} />
+        </UIExplorerBlock>
       </UIExplorerPage>
     );
   },