Refactor ReceiveMessageHandler

Introduce PlainTextWriter to improve indentation handling.

lib/src/main/java/org/asamk/signal/manager/groups/GroupId.java

@@ -33,7 +33,7 @@ public abstract class GroupId {
         }
     }
 
-    public GroupId(final byte[] id) {
+    protected GroupId(final byte[] id) {
         this.id = id;
     }