fix #1840

fix #1840
If `getResources()` is returning null here I *think* it's because the service hasn't been created yet. Perhaps `context.getResources()` would be more reliable? Also, `getResources()` is called in this same routine [here](https://github.com/wordpress-mobile/WordPress-Android/blob/issue/1840-npe-gcmintentservice/WordPress/src/main/java/org/wordpress/android/GCMIntentService.java#L178) and [here](https://github.com/wordpress-mobile/WordPress-Android/blob/issue/1840-npe-gcmintentservice/WordPress/src/main/java/org/wordpress/android/GCMIntentService.java#L213). Do we need to handle those cases, too?