Fix conversation item and data source memory leaks.

app/src/main/java/org/thoughtcrime/securesms/conversation/ConversationDataSource.java

@@ -49,7 +49,10 @@ class ConversationDataSource extends PositionalDataSource<MessageRecord> {
       }
     };
 
-    invalidator.observe(this::invalidate);
+    invalidator.observe(() -> {
+      invalidate();
+      context.getContentResolver().unregisterContentObserver(contentObserver);
+    });
 
     context.getContentResolver().registerContentObserver(DatabaseContentProviders.Conversation.getUriForThread(threadId), true, contentObserver);
   }

app/src/main/java/org/thoughtcrime/securesms/conversation/ConversationItem.java

@@ -377,6 +377,9 @@ public class ConversationItem extends LinearLayout implements BindableConversati
     if (recipient != null) {
       recipient.removeForeverObserver(this);
     }
+    if (conversationRecipient != null) {
+      conversationRecipient.removeForeverObserver(this);
+    }
   }
 
   public MessageRecord getMessageRecord() {