Defaulting timestamp for notifications on Android 7(Nougat)

* This is on by default on older version of android.
* Issue #236

Author: jkasten2

OneSignalSDK/onesignal/src/main/java/com/onesignal/GcmBroadcastReceiver.java

@@ -100,6 +100,7 @@ private static ProcessedBundleResult processOrderBroadcast(Context context, Inte
 
       Intent intentForService = new Intent();
       intentForService.putExtra("json_payload", NotificationBundleProcessor.bundleAsJSONObject(bundle).toString());
+      intentForService.putExtra("timestamp", System.currentTimeMillis() / 1000L);
       intentForService.setComponent(new ComponentName(context.getPackageName(),
                                                       GcmIntentService.class.getName()));
       startWakefulService(context, intentForService);

OneSignalSDK/onesignal/src/main/java/com/onesignal/NotificationBundleProcessor.java

@@ -368,6 +368,7 @@ static ProcessedBundleResult processBundle(Context context, final Bundle bundle)
       Intent overrideIntent = NotificationExtenderService.getIntent(context);
       if (overrideIntent != null) {
          overrideIntent.putExtra("json_payload", bundleAsJSONObject(bundle).toString());
+         overrideIntent.putExtra("timestamp", System.currentTimeMillis() / 1000L);
          WakefulBroadcastReceiver.startWakefulService(context, overrideIntent);
          result.hasExtenderService = true;
          return result;