Fix crash if recipient appears multiple times in group update description.

Without starting from start index, if the same recipient appears
multiple times in the recipient list, this function will crash.

app/src/main/java/org/thoughtcrime/securesms/database/model/GroupsV2UpdateMessageProducer.java

@@ -871,7 +871,7 @@ final class GroupsV2UpdateMessageProducer {
 
     for (RecipientId recipientId : recipientIds) {
       String placeholder      = makePlaceholder(recipientId);
-      int    placeHolderStart = template.indexOf(placeholder);
+      int    placeHolderStart = template.indexOf(placeholder, startIndex);
       String beforeChunk      = template.substring(startIndex, placeHolderStart);
 
       builder.append(beforeChunk);