Add latch to ensure application messenger is set.

There are two ways that messages can reach the StatusService,
application events (which are handled via `handleEvent`) and
signal events (which are handled via `signalEvent`).

If a signal event fires before an application event, it still
uses handleEvent logic, and handleEvent will try to reply to
the message. If the `applicationMessenger` isn't set, the reply
fails and the react native app is never told that the status
node is running, which is why the app gets no messages.

I've added a CountDownLatch, forcing the event handler to wait
for the applicationMessenger to be set before replying to any
messages.

Tested by injecting a long wait into `handleMessage` and observing
the same failure we see in SauceLabs runs.

modules/react-native-status/android/src/main/java/im/status/ethereum/module/StatusService.java

@@ -5,6 +5,7 @@ import android.content.Intent;
 import android.os.*;
 import android.support.annotation.Nullable;
 import android.util.Log;
+import java.util.concurrent.CountDownLatch;
 
 import java.lang.ref.WeakReference;
 
@@ -37,6 +38,8 @@ public class StatusService extends Service {
         }
     }
 
+    private static CountDownLatch applicationMessengerIsSet = new CountDownLatch(1);
+
     private final Messenger serviceMessenger = new Messenger(new IncomingHandler(this));
 
     private static Messenger applicationMessenger = null;
@@ -44,6 +47,7 @@ public class StatusService extends Service {
     private boolean handleMessage(Message message) {
         Log.d(TAG, "Received service message." + message.toString());
         applicationMessenger = message.replyTo;
+	applicationMessengerIsSet.countDown();
 
         return true;
     }
@@ -56,7 +60,12 @@ public class StatusService extends Service {
 
         Message replyMessage = Message.obtain(null, 0, 0, 0, null);
         replyMessage.setData(replyData);
-        sendReply(applicationMessenger, replyMessage);
+	try {
+	    applicationMessengerIsSet.await();
+	    sendReply(applicationMessenger, replyMessage);
+	} catch(InterruptedException e) {
+	    Log.d(TAG, "Interrupted during event signalling.");
+	}
     }
 
     @Nullable