Update dev menu to keep track of element inspector.

Summary:
This PR makes the android dev menu consistent with iOS where toggling the inspector will update the button label accordingly.
Closes https://github.com/facebook/react-native/pull/4628

Reviewed By: svcscm

Differential Revision: D2740549

Pulled By: mkonicek

fb-gh-sync-id: 02458d09b84e1592dcf245290ee2bbfb2863060d

ReactAndroid/src/main/java/com/facebook/react/modules/debug/DeveloperSettings.java

@@ -28,4 +28,9 @@ public interface DeveloperSettings {
    * @return Whether dev mode should be enabled in JS bundles.
    */
   boolean isJSDevModeEnabled();
+
+  /**
+   * @return Whether element inspector is enabled.
+   */
+  boolean isElementInspectorEnabled();
 }