Temporarily support IncomingMessage.online (#404)

iOS versions prior to 5.5.0.7 send `online` on `IncomingMessage`, rather
than on the top-level entity. This adds a temporary server-side adaptation,
to prevent client-side issues, like persistent typing indicators.

service/src/main/java/org/whispersystems/textsecuregcm/entities/IncomingMessage.java

@@ -32,6 +32,9 @@ public class IncomingMessage {
   @JsonProperty
   private long   timestamp; // deprecated
 
+  @JsonProperty
+  private Boolean online; // use IncomingMessageList.online - this is a temporary adaptation for older clients
+
   public String getDestination() {
     return destination;
   }
@@ -60,4 +63,7 @@ public class IncomingMessage {
     return content;
   }
 
+  public Boolean isOnline() {
+    return online;
+  }
 }