Ensure we only have one IncomingMessageObserver.

We saw a worrisome log that implied there may be a situation where
there's two IncomingMessageObservers. I can't see how that would happen,
but this is a failsafe to prevent that from happening.

app/src/main/java/org/thoughtcrime/securesms/messages/IncomingMessageObserver.java

@@ -37,6 +37,7 @@ import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicInteger;
 
 public class IncomingMessageObserver {
 
@@ -45,6 +46,8 @@ public class IncomingMessageObserver {
   public  static final  int FOREGROUND_ID            = 313399;
   private static final long REQUEST_TIMEOUT_MINUTES  = 1;
 
+  private static final AtomicInteger INSTANCE_COUNT = new AtomicInteger(0);
+
   private static SignalServiceMessagePipe pipe             = null;
   private static SignalServiceMessagePipe unidentifiedPipe = null;
 
@@ -59,6 +62,10 @@ public class IncomingMessageObserver {
   private volatile boolean terminated;
 
   public IncomingMessageObserver(@NonNull Application context) {
+    if (INSTANCE_COUNT.incrementAndGet() != 1) {
+      throw new AssertionError("Multiple observers!");
+    }
+
     this.context                    = context;
     this.networkAccess              = ApplicationDependencies.getSignalServiceNetworkAccess();
     this.decryptionDrainedListeners = new CopyOnWriteArrayList<>();
@@ -160,6 +167,8 @@ public class IncomingMessageObserver {
   }
 
   public void terminateAsync() {
+    INSTANCE_COUNT.decrementAndGet();
+
     SignalExecutors.BOUNDED.execute(() -> {
       Log.w(TAG, "Beginning termination.");
       terminated = true;