Fix edit message date header showing wrong date.

app/src/main/java/org/thoughtcrime/securesms/database/model/MessageRecord.java

@@ -741,6 +741,17 @@ public abstract class MessageRecord extends DisplayRecord {
     return originalMessageId;
   }
 
+  /**
+   * Get the timestamp of a message, shown in the conversation. This is used
+   * for example when showing the conversation date separators.
+   */
+  public long getConversationTimestamp() {
+    if (isEditMessage()) {
+      return getDateReceived();
+    }
+    return getDateSent();
+  }
+
   public int getRevisionNumber() {
     return revisionNumber;
   }